diff --git a/macros/gr_pwin32.m4 b/macros/gr_pwin32.m4
index 56273c0da..c123e052e 100644
--- a/macros/gr_pwin32.m4
+++ b/macros/gr_pwin32.m4
@@ -73,6 +73,7 @@ extern char * optarg;
 extern int optind, opterr, optopt;
 #endif
 #ifndef HAVE_GETOPT_LONG
+struct option;
 int getopt_long (int argc, char * const argv[], const char * optstring,
 			const struct option * longopts, int * longindex);
 #endif