find qmakespecs in source directory of shadow built qt

qt 5.2 does not copy the qmakespecs to the build directory any more.
a fallback to the /get property variant ensures that other build types
continue to work. consequently, this has no effect whatsoever on
installed qt versions.

(based on qtbase/214b55d5d4c8fdf975b3c36f69bac4b07d8d39d0)

Task-number: QTCREATORBUG-9572
Change-Id: I9edac11f8997fcb0594d0a67419d4733dd4ed86b
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>