Disable X11 "deprecated declaration" warning

... for GCC >= 4.6 and clang compilers.

This annoying warning exists on X11 (Linux/Unix/macOS) platforms
because we need to use XKeycodeToKeysym() although it is deprecated.
See STR 2913 for discussion and details.

src/Fl_x.cxx

@@ -1254,14 +1254,36 @@ static long getIncrData(uchar* &data, const XSelectionEvent& selevent, long lowe
   return (long)total;
 }
 
-/* Internal function to reduce "deprecated" warnings for XKeycodeToKeysym().
+/*
-   This way we get only one warning. The option to use XkbKeycodeToKeysym()
+  Internal function to reduce "deprecated" warnings for XKeycodeToKeysym().
-   instead would not help much - see STR #2913 for more information.
+  This way we get at most one warning. The option to use XkbKeycodeToKeysym()
+  instead would not help much - see STR #2913 for more information.
+
+  Update (Jan 31, 2020): disable "deprecated declaration" warnings in
+  this function for GCC >= 4.6 and clang (all versions) to get rid of
+  these warnings at least for current GCC and clang compilers.
+
+  Note: '#pragma GCC diagnostic push' needs at least GCC 4.6.
 */
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+
+#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 5))
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+#endif
+
 static KeySym fl_KeycodeToKeysym(Display *d, KeyCode k, unsigned i) {
   return XKeycodeToKeysym(d, k, i);
 }
 
+#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 5))
+#pragma GCC diagnostic pop
+#endif
+
+#pragma clang diagnostic push
+
 #if USE_XRANDR
 static void react_to_screen_reconfiguration() {
 #if USE_XFT