configure: Make GNUC visibility attribute test more reliable.

Using "int main (int argc, char** argv)" in this test causes GCC to
issue two warnings about unused variable if CFLAGS envvar has
-Wunused-parameter (or just -Wextra). Those warnings are not related
to the attribute checking but they can make the test fail anyway.

configure.ac

@@ -847,7 +847,7 @@ __attribute__ ((visibility ("default")))
      f_default (void)
 {
 }
-int main (int argc, char **argv)
+int main (void)
 {
 	f_hidden();
 	f_internal();