cmake-gui: Reference LGPLv2.1 when redistributing Qt

Download http://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt and place
it as Licenses/LGPLv2.1.txt in our source tree.  When building cmake-gui,
use option CMake_GUI_DISTRIBUTE_WITH_Qt_LGPL to enable notification
in the "About" dialog of how the distribution of Qt is licensed.
Install the license file as ${CMAKE_ROOT}/Licenses/LGPLv2.1.txt so that
the dialog can display a path to it.