ENH: Tweak how CPack package variables can be set.

This commit introduces a convenience macro named "slicer_cpackset"
allowing to set a given CPack variable to either "Slicer<cpack_varname>"
or "<${Slicer_MAIN_PROJECT}_APPLICATIONNAME><cpack_varname>".

A configure time message displaying the package properties has also
been added:

[...]
-- Setting CPACK_PACKAGE_NAME to 'Slicer'
-- Setting CPACK_PACKAGE_VENDOR to 'NA-MIC'
-- Setting CPACK_PACKAGE_DESCRIPTION_SUMMARY to 'Medical Visualization and Processing Environment for Research'
-- Setting CPACK_PACKAGE_DESCRIPTION_FILE to '/home/jchris/Projects/Slicer-2/README.txt'
-- Setting CPACK_RESOURCE_FILE_LICENSE to '/home/jchris/Projects/Slicer-2/License.txt'
-- Setting CPACK_PACKAGE_VERSION_MAJOR to '4'
-- Setting CPACK_PACKAGE_VERSION_MINOR to '4'
-- Setting CPACK_PACKAGE_VERSION_PATCH to '0'
-- Setting CPACK_PACKAGE_VERSION to '4.4.0-2015-01-23'
-- Setting CPACK_PACKAGE_INSTALL_DIRECTORY to 'Slicer 4.4.0-2015-01-23'
[...]

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23927 3bd1e089-480b-0410-8dfb-8563597acbee