COMP: Change slicer_external_update to ${PROJECT_NAME}_EP_UPDATE_IF_GREATER_288

The new variable name better represents the intent of this variable
and it is more appropriate for the many other non-slicer
projects that use derivatives of these External_*.cmake
files.

Contributed by Hans Johnson

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21667 3bd1e089-480b-0410-8dfb-8563597acbee