DEV: Add static modifier to inline declarations.
It is currently impossible to build a debug version of Pandas on Mac OS, since some of the C function declarations are declared as `inline`, and compiling in debug mode turns off optimizations and hence inlining. While this could be solved by adding a compile argument to `setup.py` to use the C89 behavior for inline as in #10510, I believe an ultimately cleaner solution may consist of declaring inlined functions as `static`, so that the code is properly C99-compliant.

For the implementation, I added the `static` keyword to the definitions of `PANDAS_INLINE` and `P_INLINE` (and removed duplicate occurrences of `static` from some function declarations), so that future implementations that use those defines are automatically compliant. This is slightly different from e.g. Cython, which defines `CYTHON_INLINE` as `inline` and declares everything as `static CYTHON_INLINE`.

I think this provides an alternative solution for #10510, but I haven't tested this yet (currently downloading a FreeBSD vm over crummy internet).
 Makes sense to me. Reviewed lgtm if the build passes. 
 I ended up removing the conditional flag `-fgnu89-inline` on FreeBSD, since the addition of `static` makes it unnecessary. Building succeeds on my copy of FreeBSD 10.2, and the tests more or less run (there are some failures but they don't look related).
 pls add a whatsnew note
and comment on that other issue for them to try this PR (just to confirm)
 Done. Thanks guys for the quick feedback.
 @jvkersch thank you sir!
