xconfig: use pkgconfig to find moc

Various schemes exist to allow parallel installations of multiple major
versions of Qt (4.x with the previous 3.x and/or the upcoming 5.x).
QtCore.pc includes a moc_location variable which should be a more reliable
way to find moc.

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>