Add.

m4/mbrtowc.m4

@@ -0,0 +1,18 @@
+#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
+])