QT_USE_FAST_OPERATOR_PLUS has been deprecated in Qt 4.8, in favour of
QT_USE_QSTRINGBUILDER. The latter also covers QByteArray.

Qt5 headers still support QT_USE_FAST_OPERATOR_PLUS,
as subset of QT_USE_QSTRINGBUILDER as it was introduced in Qt4.8,
but not as documented build flag. As QT_USE_QSTRINGBUILDER is already set
in this project, we can drop it.

GIT_SILENT