Add license file to cpack distribution
Currently when creating either a source or binary distribution with cpack using the TGZ generator, no license files are included.
This might be different for other generators.
A fix to this issue should also include the COPYING-OTHER added by https://gerrit.gromacs.org/#/c/1042/
Target set for 4.6 and high because this has to be fixed before the release to comply with the licenses. I assume we are planning to use cpack for the release. It is possible for the 4.6 release to manually add the required licenses files if we don't want to fix the cpack configuration before then.