Conditionally ignore some GCC warnings

Some GCC versions don't know about some warnings, and they complain
that we're ignoring unknown warnings. So we try to ignore the warning
based on the GCC version.

rts/IOManager.c

@@ -74,7 +74,9 @@
  * simply disable these two warnings for the whole file.
  */
 #pragma GCC diagnostic push
+#if __GNUC__ >= 5
 #pragma GCC diagnostic ignored "-Wsuggest-attribute=noreturn"
+#endif
 #pragma GCC diagnostic ignored "-Wmissing-noreturn"
 #pragma GCC diagnostic ignored "-Wunused-parameter"