count-one-bits: better 'inline'

* lib/count-one-bits.c: New file.
* lib/count-one-bits.h (COUNT_ONE_BITS_INLINE):
New macro.  Replace all uses of 'static inline' with it.
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* m4/count-one-bits.m4 (gl_COUNT_ONE_BITS):
Do not require AC_C_INLINE.
* modules/count-one-bits (Files, lib_SOURCES):
Add lib/count-one-bits.c.
(Depends-on): Add extern-inline.