Set QT_USE_FAST_OPERATOR_PLUS ourselves

While QT_USE_QSTRINGBUILDER, as set from KDEFrameworkCompilerSettings,
is explicitely unset again, that flag's still effective (despite no
longer official since Qt 4.8) predecessor QT_USE_FAST_OPERATOR_PLUS
has been not. Though until recently QT_USE_FAST_OPERATOR_PLUS was set
as well in KDEFrameworkCompilerSettings, but now is no longer, being
a subset of QT_USE_QSTRINGBUILDER and no official flag in Qt5.

As KDELibs4Support had been already adapted to build fine with the
feature subset enabled by QT_USE_FAST_OPERATOR_PLUS (and actually failed
to build with it not set), setting it ourselves now, to restore the old
build condition.