I thought about that for a bit and I'm really not sure what the sanest
way is.
It could get the value from TRACK_ROOT, then prepend INSTALL_ROOT
because it should be built against the one in INSTALL_ROOT, whatever it
later uses. I figured with what I added, it should work in most cases,
even in the case a user installs his own qt, as it prefers the set value
of QTDIR. The whole TRACK_ROOT/INSTALL_ROOT business is still not really
100% clear, we should take a very close look at the various use cases
once we have sorcery with stage_root support.
As it is, my primary concern was to get it working in the use-case that
a normal system without INSTALL_ROOT or TRACK_ROOT set casts mythtv
without having qt-x11 installed, so that's what I fixed ;)