look for mkspecs in the project root, not next to every project

look for mkspecs in the project root, not next to every project

qmake would look for mkspecs/ in the directory containing the current
project file. this makes completely no sense with recursive projects:
a) nobody would make per-project specs and b) specs meant to be global
would not be found.

consequently, we look for a project root when starting qmake and use
only that directory.
if .qmake.cache is found/set, we assume that to be the project root.
otherwise, we search for mkspecs/ the same way we search for the cache -
just to up until we find one or hit the root. if we are shadow-building,
search the build dir as well.

Change-Id: Ie66b189a40c21203d956e681cbef44a89f98cd17
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>