Add the infrastructure to compile Qt with -Werror

This is enabled only for -developer-builds and only for certain
compiler-version combinations that are in a whitelist.

It also requires each library, plugin or tool to declare whether it is
supposedly clean of warnings. When most targets are clean, we can
consider inverting.

Change-Id: I17b5c4e45aee5078f9788e846a45d619c144095a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>