--- a/m4/wmemset.m4
+++ b/m4/wmemset.m4
@@ -1,4 +1,4 @@
-# wmemset.m4 serial 2
+# wmemset.m4 serial 3
 dnl Copyright (C) 2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -7,8 +7,26 @@
 AC_DEFUN([gl_FUNC_WMEMSET],
 [
   AC_REQUIRE([gl_WCHAR_H_DEFAULTS])
-  AC_CHECK_FUNCS_ONCE([wmemset])
-  if test $ac_cv_func_wmemset = no; then
+  dnl We cannot use AC_CHECK_FUNCS here, because the MSVC 9 header files
+  dnl provide this function as an inline function definition.
+  AC_CACHE_CHECK([for wmemset], [gl_cv_func_wmemset],
+    [AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM([[
+/* Tru64 with Desktop Toolkit C has a bug: <stdio.h> must be included before
+   <wchar.h>.
+   BSD/OS 4.0.1 has a bug: <stddef.h>, <stdio.h> and <time.h> must be included
+   before <wchar.h>.  */
+#include <stddef.h>
+#include <stdio.h>
+#include <time.h>
+#include <wchar.h>
+]],
+          [[return ! wmemset ((wchar_t *) 0, (wchar_t) ' ', 0);]])
+       ],
+       [gl_cv_func_wmemset=yes],
+       [gl_cv_func_wmemset=no])
+    ])
+  if test $gl_cv_func_wmemset = no; then
     HAVE_WMEMSET=0
   fi
 ])