Enable -Werror for all of qtbase

This change applies to all qtbase modules, tools and applications. It
does not apply to examples or unit tests.

This change complements the changes done to mkspecs/qt_common.prf
(especially 043b80919747676c2df4b4423ed5950583233d30 and
ebfd85a499a4382ace09d443b1f35cd6b1848af6). It enables -Werror checking
in qtbase for GCC 4.6-4.8, Apple clang 4.0-4.2 and ICC 13.0-14.0.

Change-Id: I6d29a7a1b3716960a149409f551363385991714c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>