qt5: expose a QT5_QMAKE variable for other Qt5 packages

Much like package/qt/qt.mk exposes a QT_QMAKE variable that other
qmake-based Qt4 libraries/applications can use in their .mk file, this
commit adds a QT5_QMAKE variable to allow the same for Qt5 packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>