Cleanup: compiler warnings with clang

* Mark either all or no class methods with override
* Don't use zero sized array since it has a different size in C and C++.
  Using a little more memory here is not significant.
* Don't use deprecated mechanism to mark private GSet members in clang
  just like we don't for MSVC, it warns even for simple zero initialization.

source/blender/blenlib/BLI_ghash.h

@@ -234,7 +234,7 @@ void *BLI_gset_pop_key(GSet *gs, const void *key) ATTR_WARN_UNUSED_RESULT;
 /* so we can cast but compiler sees as different */
 typedef struct GSetIterator {
   GHashIterator _ghi
-#ifdef __GNUC__
+#if defined(__GNUC__) && !defined(__clang__)
       __attribute__((deprecated))
 #endif
       ;