COMP: handle case where TCL is disabled but EM is set to ON.

If you have first configured with TCL on, then EMSegment is on
by default too.  But turning off TCL does not turn off EMSegment,
so the linker was failing to find the EMSegment logic library.

This patch is consistent with the rest of the logic in the
CMakeLists, but longer term EMSegment should be explicitly
disabled if it doesn't have the right prerequisites.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22793 3bd1e089-480b-0410-8dfb-8563597acbee