COMP: SlicerExtensionCPack: Use proper variable evaluation

This commit should fix UKFTractography extension packaging, it is
currently failing because "UKFTractography_SUPERBUILD" is an option
and checking if it is defined always return TRUE.

It also fixes a regression introduced in r25181 (BUG: Extension
packaging: For SuperBuild extension, ensure fixup happen last)

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

From: Isaiah Norton <inorton@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25303 3bd1e089-480b-0410-8dfb-8563597acbee