The Xcode and SDK settings are expensive to resolve, as we're using
system() calls to resolve them. We now try to detect the presence of
a .qmake.cache file (and inform the user that creating one would be
a good idea), and use the file to cache the various settings after
resolving them.

The Xcode logic had to be moved form xcode.conf as part of the mkspec,
into default_pre/post.prf, so that we could cache() the resolved values.

Task-number: QTBUG-30586
Change-Id: Ib5368cfee6f7e4a4a33f6be70d0e20d96896fe56
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>