compatibility issues with master branch cython
On the cython master branch the generated c code has unused  functions which fails the build with:

```

  gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -fno-semantic-interposition -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -march=x86-64 -mtune=generic -O3 -pipe -fno-plt -fPIC -DNPY_NO_DEPRECATED_API=0 -I./pandas/_libs -Ipandas/_libs/src/klib -I/home/tcaswell/.virtualenvs/sys38/lib/python3.8/site-packages/numpy/core/include -I/home/tcaswell/.virtualenvs/sys38/include -I/usr/include/python3.8 -c pandas/_libs/algos.c -o build/temp.linux-x86_64-3.8/pandas/_libs/algos.o -Werror
    pandas/_libs/algos.c:87270:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_762__defaults__’ defined but not used [-Werror=unused-function]
    87270 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_762__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:78766:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_744__defaults__’ defined but not used [-Werror=unused-function]
    78766 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_744__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:71391:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_726__defaults__’ defined but not used [-Werror=unused-function]
    71391 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_726__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:62479:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_724__defaults__’ defined but not used [-Werror=unused-function]
    62479 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_724__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:56193:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_678__defaults__’ defined but not used [-Werror=unused-function]
    56193 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_678__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:50908:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_632__defaults__’ defined but not used [-Werror=unused-function]
    50908 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_632__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:40806:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_586__defaults__’ defined but not used [-Werror=unused-function]
    40806 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_586__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:34476:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_540__defaults__’ defined but not used [-Werror=unused-function]
    34476 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_540__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:29147:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_494__defaults__’ defined but not used [-Werror=unused-function]
    29147 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_494__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:19248:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_448__defaults__’ defined but not used [-Werror=unused-function]
    19248 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_448__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    pandas/_libs/algos.c:12162:18: error: ‘__pyx_pf_6pandas_5_libs_5algos_446__defaults__’ defined but not used [-Werror=unused-function]
    12162 | static PyObject *__pyx_pf_6pandas_5_libs_5algos_446__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cc1: all warnings being treated as errors
    error: command 'gcc' failed with exit status 1
```

and similar warnings from other modules (this just happens to be the first one).

If I remove the `"-Werror"` it seems to build ok.

Not sure if this should be reported here at cython.  I will likely try to bisect this to the cython change later today.