Passing a URL via defines breaks when trying to "stringify" it.
It would always stop at the first slash.
Instead use a qmake variable directly. That has the additional advantage that
the value isn't passed on each and every compiler command line.
Change-Id: I97a297fbcdee5ea03eb6fa08181c95b1ad2de767
Reviewed-by: Cristian Adam <cristian.adam@qt.io>