The common config file doesn't add -Werror directly to QMAKE_CXXFLAGS anymore, it sets the warnings_are_errors variable so that it is correctly handled by different compilers (not just GCC). That allows to remove the -Werror=deprecated-declarations flag since warnings_are_errors defines it for us.
Removed the DEFINES += QT_USE_QSTRINGBUILDER lines since it's already handled by qt_module.prf.
Set the -Wsuggest-override warning to QMAKE_CXXFLAGS_WARN_ON instead of QMAKE_CXXFLAGS directly.