* lib/stdlib.c: New file.
* modules/stdlib (Files, lib_SOURCES): Add it.
(Depends-on): Add extern-inline.
* lib/stdlib.in.h: Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
(_GL_STDLIB_INLINE): New macro.
(gl_MB_CUR_MAX): Make it _GL_STDLIB_INLINE not static inline,
so that it can be called from extern inline functions.
Redo implementation to help GCC better optimize common expressions
like (MB_CUR_MAX == 1).