COMP: Ensure 'superbuild_include_once' works independently of caller scope.

By using GLOBAL property instead of a local variable, the file will
be properly "include guarded" even if included from within a function.

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22751 3bd1e089-480b-0410-8dfb-8563597acbee