Allow "Out-Of-Source" Build (Original patch by yarrouye)

... separated the "out-of-source" patch from yves from original pull
request
... the patch allows to use e.g. QTCreators "Shadow-Build" feature so
that GC is build not in source, but in a different directory
.. this is useful when using QTCreator and switch between different QT
versions / 32/64 bit builds - since every build resides in a separate
directory from GC source

-- since the original commit (at least from GitHub view) is shown in a
mixed pull request with other commits - this is a seperated version of
the single commit
Merge pull request #1205 from Joern-R/creator

Allow "Out-Of-Source" Build (Original patch by yarrouye)