Port quotearg fixes from tar 1.13.24.

Port quotearg fixes from tar 1.13.24.
* lib/quotearg.c: BSD/OS 4.1 wchar.h requires FILE and struct
tm to be declared.
(HAVE_MBSINIT): Undef if !HAVE_MBRTOWC.
(mbsinit): Define to 1 if !defined mbsinit && !HAVE_MBSINIT.
* m4/Makefile.am (EXTRA_DIST): Add mbrtowc.m4.
* m4/mbrtowc.m4: New file.
* m4/prereq.m4 (jm_PREREQ_QUOTEARG): Check for mbsinit and stddef.h.
Use jm_FUNC_MBRTOWC instead of AC_CHECK_FUNCS(mbrtowc).