KDE_INSTALL_USE_QT_SYS_PATHS as default

[Aleix Pol]

Hi,
It's been on some occasions already that I've seen reports of people
trying to get some KDE application installed and complaining (mostly
the ones based on QML). The reason is that KDE is assuming the
application is being installed into its own prefix by default.

I've been wondering that maybe we shouldn't? Or maybe we could try to
detect if it's the same prefix as Qt and in that case install things
together.

Opinions?
Aleix