COMP: Associate license file with installer/package

Since commit r21550 (Add support for external projects passed as
configuration parameter), most of CPack variable can be overridden
when building an application that depends on Slicer as a "framework".

While establishing the indirection, a typo as been introduced and the wrong
variable name was used. This commit restore the appropriate behavior.

See http://na-mic.org/Bug/view.php?id=2961
Fixes #2961

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21729 3bd1e089-480b-0410-8dfb-8563597acbee