find qmakespecs in source directory of shadow built qt

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>
(cherry picked from qtcreator/958672e4607acf654b0fe5f35a385e95a3042f23)