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).

aclocal.m4

@@ -788,21 +788,41 @@ AC_DEFUN(jm_FUNC_REALLOC,
   fi
 ])
 
-#serial 1
+#serial 2
 
 dnl These are the prerequisite macros for files in the lib/
 dnl directories of Bison.
 
 AC_DEFUN([jm_PREREQ_QUOTEARG],
 [
-  AC_CHECK_FUNCS(isascii iswprint mbrtowc)
-  AC_CHECK_HEADERS(limits.h stdlib.h string.h wchar.h wctype.h)
+  AC_CHECK_FUNCS(isascii iswprint mbsinit)
+  jm_FUNC_MBRTOWC
+  AC_CHECK_HEADERS(limits.h stddef.h stdlib.h string.h wchar.h wctype.h)
   AC_HEADER_STDC
   AC_C_BACKSLASH_A
   AC_MBSTATE_T
   AM_C_PROTOTYPES
 ])
 
+#serial 4
+
+dnl From Paul Eggert
+
+AC_DEFUN([jm_FUNC_MBRTOWC],
+[
+  AC_CACHE_CHECK([whether mbrtowc and mbstate_t are properly declared],
+    jm_cv_func_mbrtowc,
+    [AC_TRY_LINK(
+       [#include <wchar.h>],
+       [mbstate_t state; return ! (sizeof state && mbrtowc);],
+       jm_cv_func_mbrtowc=yes,
+       jm_cv_func_mbrtowc=no)])
+  if test $jm_cv_func_mbrtowc = yes; then
+    AC_DEFINE(HAVE_MBRTOWC, 1,
+      [Define to 1 if mbrtowc and mbstate_t are properly declared.])
+  fi
+])
+
 #serial 3
 
 dnl From Paul Eggert.