Cache Xcode and SDK settings in .qmake.cache if it exists

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>