Check for mbstate_t, too.

m4/mbrtowc.m4

@@ -4,16 +4,16 @@ dnl From Paul Eggert
 
 AC_DEFUN(jm_FUNC_MBRTOWC,
 [
-  AC_MSG_CHECKING([for mbrtowc])
+  AC_MSG_CHECKING([whether mbrtowc and mbstate_t are declared])
   AC_CACHE_VAL(jm_cv_func_mbrtowc,
     [AC_TRY_LINK(
        [#include <wchar.h>],
-       [return !mbrtowc;],
+       [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_MSG_RESULT(yes)
     AC_DEFINE(HAVE_MBRTOWC, 1,
-      [Define to 1 if mbrtowc exists and is properly declared.])
+      [Define to 1 if mbrtowc and mbstate_t are properly declared.])
   fi
 ])