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fixing copyright and license
Fix CREATE_YARPSCOPE option on windows

FindGtkMM reports the wrong version on windows, since the check was
for a problem strictly related to debian etch, for now we do this check
only on UNIX.
But FindGtkMM should definitely be fixed.

cherry-picked from r9643
Bump YARP_VERSION_TWEAK to 1
tagging version 2.3.20.1