this is cleaner than having it parse qmake project files.
the only remaining built-in version extraction is the fallback to
qglobal.h needed for bootstrapping.

as a "side effect", this fixes the build of modules with mismatched
versions centralized in .qmake.conf, as this was simply not handled so
far.

the -mkspecsdir syncqt option goes away, as there is no use case for it
any more.

Task-number: QTBUG-29838
Change-Id: I6912a38f0e93a26bc267a9e3d738506fd3ad431b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>