diff -Naur a/gl/m4/mbrtowc.m4 b/gl/m4/mbrtowc.m4
--- a/gl/m4/mbrtowc.m4	2015-12-24 10:41:48.000000000 -0600
+++ b/gl/m4/mbrtowc.m4	2016-09-24 13:02:03.404000000 -0500
@@ -569,7 +569,7 @@
            int
            main (void)
            {
-             return ! mbrtowc (&wc, "", 0, &mbs) == (size_t) -2;
+             return mbrtowc (&wc, "", 0, &mbs) != (size_t) -2;
            }]])],
         [gl_cv_func_mbrtowc_empty_input=yes],
         [gl_cv_func_mbrtowc_empty_input=no],