(jm_PREREQ_MBSWIDTH): Check for wcwidth declaration.

m4/mbswidth.m4

@@ -1,4 +1,4 @@
-#serial 1
+#serial 2
 
 dnl autoconf tests required for use of mbswidth.c
 dnl From Bruno Haible.
@@ -9,5 +9,11 @@ AC_DEFUN(jm_PREREQ_MBSWIDTH,
   AC_REQUIRE([AM_C_PROTOTYPES])
   AC_CHECK_HEADERS(limits.h stdlib.h string.h wchar.h wctype.h)
   AC_CHECK_FUNCS(isascii iswprint mbrtowc wcwidth)
+  headers='
+#     if HAVE_WCHAR_H
+#      include <wchar.h>
+#     endif
+'
+  AC_CHECK_DECLS([wcwidth], , , $headers)
   AC_MBSTATE_T
 ])