Do not remove obsolete enum values for gcc and clang

Do not try to work around the gcc/clang limitations with external variables
since some people have some concern that it may break the binary compatibility
due to the underlying representation layout.

Talk to upstream (gcc) if a warning for this is desired. It is out of scope for
us. C++14 will help with the [[deprecated]] attribute, but that is light years
ahead.

Task-number: QTBUG-35354
Change-Id: I1c4e0fb950469adceca85147084fa896503de78b
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>