QT_USE_QSTRINGBUILDER to fix source compatibility

QT_USE_QSTRINGBUILDER to fix source compatibility

In 4.8 we added support for using StringBuilder with QByteArray.
But this is breaking source compatibility for people that used
QT_USE_FAST_OPERATOR_PLUS in Qt 4.7. So we introduce a new macro

Notice that QT_USE_FAST_CONCATENATION was not working without
QT_USE_FAST_OPERATOR_PLUS, so we remove the checking of that macro.

Reviewed-by: joao
(cherry picked from commit 8447f5616be731d78081f326bb9cb3f5aa9087a4)