COMP: Do not choke on extension description containing semi-colon.

This commit fixes the following error:

CMake Error at /path/to/Slicer/CMake/UseSlicerMacros.cmake:92 (string):
string sub-command LENGTH requires two arguments.
Call Stack (most recent call first):
/path/to/Slicer-build/UseSlicer.cmake:392 (slicer_setting_variable_message)
CMakeLists.txt:17 (include)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23610 3bd1e089-480b-0410-8dfb-8563597acbee