Work around declaration collisions on Minix.

Work around declaration collisions on Minix.

* m4/mbsinit.m4 (gl_FUNC_MBSINIT): If mbsinit is declared but not
defined, set REPLACE_MBSINIT.
* m4/mbrtowc.m4 (gl_FUNC_MBRTOWC): If mbrtowc is declared but not
defined, set REPLACE_MBRTOWC.
* m4/mbrlen.m4 (gl_FUNC_MBRLEN): If mbrlen is declared but not defined,
set REPLACE_MBRLEN.
* m4/mbsrtowcs.m4 (gl_FUNC_MBSRTOWCS): If mbsrtowcs is declared but not
defined, set REPLACE_MBSRTOWCS.
* m4/wcrtomb.m4 (gl_FUNC_WCRTOMB): If wcrtomb is declared but not
defined, set REPLACE_WCRTOMB.
* m4/wcsrtombs.m4 (gl_FUNC_WCSRTOMBS): If wcsrtombs is declared but not
defined, set REPLACE_WCSRTOMBS.