Build error with np_datetime_strings on OS X 10.7
Building from most recently-available source, I get the following build error (on OS X 10.7 with clang compiler):

```
gcc-4.2 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -pipe -arch x86_64 -I/Library/Python/2.7/site-packages/numpy-1.7.0.dev_3cb783e_20120425-py2.7-macosx-10.7-x86_64.egg/numpy/core/include -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c pandas/src/np_datetime_strings.c -o build/temp.macosx-10.7-x86_64-2.7/pandas/src/np_datetime_strings.o
In file included from /Library/Python/2.7/site-packages/numpy-1.7.0.dev_3cb783e_20120425-py2.7-macosx-10.7-x86_64.egg/numpy/core/include/numpy/ndarraytypes.h:1974,
                 from /Library/Python/2.7/site-packages/numpy-1.7.0.dev_3cb783e_20120425-py2.7-macosx-10.7-x86_64.egg/numpy/core/include/numpy/ndarrayobject.h:17,
                 from /Library/Python/2.7/site-packages/numpy-1.7.0.dev_3cb783e_20120425-py2.7-macosx-10.7-x86_64.egg/numpy/core/include/numpy/arrayobject.h:15,
                 from pandas/src/np_datetime_strings.c:17:
/Library/Python/2.7/site-packages/numpy-1.7.0.dev_3cb783e_20120425-py2.7-macosx-10.7-x86_64.egg/numpy/core/include/numpy/npy_deprecated_api.h:11:2: warning: #warning "Using deprecated NumPy API, disable it by #defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION"
pandas/src/np_datetime_strings.c: In function ‘get_datetime_iso_8601_strlen’:
pandas/src/np_datetime_strings.c:911: error: ‘NPY_FR_B’ undeclared (first use in this function)
pandas/src/np_datetime_strings.c:911: error: (Each undeclared identifier is reported only once
pandas/src/np_datetime_strings.c:911: error: for each function it appears in.)
pandas/src/np_datetime_strings.c:888: warning: enumeration value ‘NPY_FR_GENERIC’ not handled in switch
error: command 'gcc-4.2' failed with exit status 1
```
