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 043b809 and
ebfd85a). 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>

.qmake.conf

@@ -1,5 +1,6 @@
 load(qt_build_config)
 CONFIG += qt_example_installs
+CONFIG += warning_clean
 
 QT_SOURCE_TREE = $$PWD
 QT_BUILD_TREE = $$shadowed($$PWD)