WARN: more build warnings
e.g.: https://travis-ci.org/pandas-dev/pandas/jobs/290640499
after #17932  most are signed / unsigned comparisons

```
I/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include -I/home/travis/miniconda3/envs/pandas/include/python3.6m -c pandas/_libs/lib.c -o build/temp.linux-x86_64-3.6/pandas/_libs/lib.o -Wno-unused-function
In file included from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarraytypes.h:1809:0,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarrayobject.h:18,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/arrayobject.h:4,
 from pandas/_libs/lib.c:518:
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
 #warning "Using deprecated NumPy API, disable it by " \
 ^
pandas/_libs/lib.c: In function ‘__pyx_pw_6pandas_5_libs_3lib_51maybe_booleans_to_slice’:
pandas/_libs/lib.c:12591:15: warning: ‘__pyx_v_end’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 __pyx_t_6 = PyInt_FromSsize_t(__pyx_v_end); if (unlikely(!__pyx_t_6)) __PYX_ERR(0, 641, __pyx_L1_error)
 ^
pandas/_libs/lib.c:12233:14: note: ‘__pyx_v_end’ was declared here
 Py_ssize_t __pyx_v_end;
 ^
pandas/_libs/lib.c:12589:15: warning: ‘__pyx_v_start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 __pyx_t_7 = PyInt_FromSsize_t(__pyx_v_start); if (unlikely(!__pyx_t_7)) __PYX_ERR(0, 641, __pyx_L1_error)
 ^
pandas/_libs/lib.c:12232:14: note: ‘__pyx_v_start’ was declared here
 Py_ssize_t __pyx_v_start;
 ^
```

```
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include -I/home/travis/miniconda3/envs/pandas/include/python3.6m -c pandas/_libs/hashtable.c -o build/temp.linux-x86_64-3.6/pandas/_libs/hashtable.o -Wno-unused-function
In file included from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarraytypes.h:1809:0,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarrayobject.h:18,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/arrayobject.h:4,
 from pandas/_libs/hashtable.c:519:
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
 #warning "Using deprecated NumPy API, disable it by " \
 ^
In file included from pandas/_libs/hashtable.c:521:0:
pandas/_libs/src/klib/khash_python.h: In function ‘asint64’:
pandas/_libs/src/klib/khash_python.h:17:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
 return *(khint64_t *)(&key);
 ^
pandas/_libs/hashtable.c: In function ‘__pyx_f_6pandas_5_libs_9hashtable_13Float64Vector_extend’:
pandas/_libs/hashtable.c:4591:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (__pyx_t_2 = 0; __pyx_t_2 < __pyx_t_1; __pyx_t_2+=1) {
 ^
pandas/_libs/hashtable.c: In function ‘__pyx_f_6pandas_5_libs_9hashtable_12UInt64Vector_extend’:
pandas/_libs/hashtable.c:5479:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (__pyx_t_2 = 0; __pyx_t_2 < __pyx_t_1; __pyx_t_2+=1) {
 ^
```

```
from pandas/_libs/tslib.c:518:
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
 #warning "Using deprecated NumPy API, disable it by " \
 ^
In file included from pandas/_libs/tslib.c:528:0:
pandas/_libs/src/klib/khash_python.h: In function ‘asint64’:
pandas/_libs/src/klib/khash_python.h:17:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
 return *(khint64_t *)(&key);
 ^
pandas/_libs/tslib.c: In function ‘__pyx_pf_6pandas_5_libs_5tslib_50tz_localize_to_utc.isra.94’:
pandas/_libs/tslib.c:61709:21: warning: ‘__pyx_pybuffernd_dst_hours.diminfo[0].strides’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 __Pyx_LocalBuf_ND __pyx_pybuffernd_dst_hours;
 ^
```

```
In file included from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarraytypes.h:1809:0,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarrayobject.h:18,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/arrayobject.h:4,
 from pandas/_libs/src/datetime/np_datetime.c:22:
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
 #warning "Using deprecated NumPy API, disable it by " \
 ^
In file included from pandas/_libs/src/datetime/np_datetime.c:20:0:
/home/travis/miniconda3/envs/pandas/include/python3.6m/datetime.h:200:25: warning: ‘PyDateTimeAPI’ defined but not used [-Wunused-variable]
 static PyDateTime_CAPI *PyDateTimeAPI = NULL;
 ^
```

```
uild/temp.linux-x86_64-3.6/pandas/_libs/index.o -Wno-unused-function
In file included from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarraytypes.h:1809:0,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarrayobject.h:18,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/arrayobject.h:4,
 from pandas/_libs/index.c:518:
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
 #warning "Using deprecated NumPy API, disable it by " \
 ^
In file included from pandas/_libs/index.c:523:0:
pandas/_libs/src/klib/khash_python.h: In function ‘asint64’:
pandas/_libs/src/klib/khash_python.h:17:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
 return *(khint64_t *)(&key);
 ^
pandas/_libs/index.c: In function ‘__pyx_f_6pandas_5_libs_5index_11IndexEngine_get_loc’:
pandas/_libs/index.c:9477:13: warning: ‘__pyx_v_mid’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 __pyx_t_5 = PyInt_FromSsize_t(__pyx_v_mid); if (unlikely(!__pyx_t_5)) __PYX_ERR(0, 380, __pyx_L1_error)
 ^
pandas/_libs/index.c:9252:14: note: ‘__pyx_v_mid’ was declared here
 Py_ssize_t __pyx_v_mid;
 ^
```

```
I/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include -I/home/travis/miniconda3/envs/pandas/include/python3.6m -c pandas/_libs/interval.c -o build/temp.linux-x86_64-3.6/pandas/_libs/interval.o -Wno-unused-function
In file included from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarraytypes.h:1809:0,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/ndarrayobject.h:18,
 from /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/arrayobject.h:4,
 from pandas/_libs/interval.c:518:
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
 #warning "Using deprecated NumPy API, disable it by " \
 ^
In file included from pandas/_libs/interval.c:523:0:
pandas/_libs/src/klib/khash_python.h: In function ‘asint64’:
pandas/_libs/src/klib/khash_python.h:17:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
 return *(khint64_t *)(&key);
 ^
pandas/_libs/interval.c: In function ‘__pyx_pf_6pandas_5_libs_8interval_12IntervalTree_26get_indexer’:
pandas/_libs/interval.c:12248:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (__pyx_t_3 = 0; __pyx_t_3 < __pyx_t_2; __pyx_t_3+=1) {
 ^
pandas/_libs/interval.c: In function ‘__pyx_pf_6pandas_5_libs_8interval_12IntervalTree_28get_indexer’:
pandas/_libs/interval.c:12505:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (__pyx_t_3 = 0; __pyx_t_3 < __pyx_t_2; __pyx_t_3+=1) {
 ^
pandas/_libs/interval.c: In function ‘__pyx_pf_6pandas_5_libs_8interval_12IntervalTree_30get_indexer’:
pandas/_libs/interval.c:12762:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (__pyx_t_3 = 0; __pyx_t_3 < __pyx_t_2; __pyx_t_3+=1) {
 ^
pandas/_libs/interval.c: In function ‘__pyx_pf_6pandas_5_libs_8interval_12IntervalTree_32get_indexer’:
pandas/_libs/interval.c:13019:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (__pyx_t_3 = 0; __pyx_t_3 < __pyx_t_2; __pyx_t_3+=1) {
```