math: better 'inline'

math: better 'inline'

* lib/math.c: New file.
* lib/math.in.h (_GL_MATH_INLINE):
New macro.  Replace all uses of 'static inline' with it.
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* m4/math_h.m4 (gl_MATH_H):
Do not require AC_C_INLINE.
* modules/math (Files, lib_SOURCES):
Add lib/math.c.
(Depends-on): Add extern-inline.