Using cmake macros to create the Version.cmake file
(cherry picked from commit aa52a96f5d506b2b85d16f6b30d45ad6215bd5f9)

Conflicts:
	conf/YarpDescribe.cmake
Cleanup version stuff

* Rename YARP_GENERIC_VERSION to YARP_VERSION and define it only once
* Add YARP_VERSION_STRING
* Remove YARP_VERSION_MODIFIER
* Better handling of YARP_VERSION_TWEAK
Merge pull request #50 from drdanz/version_cleanup

 Cleanup version stuff
Restore YARP_VERSION_TWEAK
YARP 2.3.21

This time for real!