Add a version check for duplicated-branches warning

The GHashTable code ignores the duplicated-branches GCC warning, but we
need to do a compiler and version check, as either non-GCC compatible
compilers, or older versions of GCC will warn about the unknown pragma
or diagnostic.

If we don't do this while turning warnings into error, we're going to
fail the build unnecessarily.

glib/ghash.c

@@ -56,7 +56,9 @@
  *
  * ...and still compile successfully when -Werror=duplicated-branches is passed. */
 
+#if defined(__GNUC__) && __GNUC__ > 6
 #pragma GCC diagnostic ignored "-Wduplicated-branches"
+#endif
 
 /**
  * SECTION:hash_tables