GFlags: Fix usage of unused attribute

extern/gflags/README.blender

@@ -21,3 +21,6 @@ Local modifications:
   not trigger strict compiler warning.
 
 - Did the same for CommandLineFlagParser::ValidateFlags().
+
+- Ifdef-ed __attribute((unused)) in gflags.h.
+  This file is compile-time configurable in upstream, so can not avoid change here.

extern/gflags/src/gflags/gflags.h

@@ -285,7 +285,11 @@ class GFLAGS_DLL_DECL FlagSaver {
 
   FlagSaver(const FlagSaver&);  // no copying!
   void operator=(const FlagSaver&);
-}__attribute((unused));
+}
+#ifdef __GNUC__
+__attribute((unused))
+#endif
+;
 
 // --------------------------------------------------------------------
 // Some deprecated or hopefully-soon-to-be-deprecated functions.