DEV: Add static modifier to inline declarations.

[jvkersch]

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.

[jvkersch]

I think this provides an alternative solution for #10510, but I haven't tested this yet (currently downloading a FreeBSD vm over crummy internet).

[wesm]

Makes sense to me. Reviewed lgtm if the build passes.

[jvkersch]

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).

[jreback]

pls add a whatsnew note
and comment on that other issue for them to try this PR (just to confirm)

[jvkersch]

Done. Thanks guys for the quick feedback.

[jreback]

@jvkersch thank you sir!