Notes:

(1) If Subversion is not found, EMSegment module will not be built
because its sources are still hosted on SVN.

(2) while this commit removes the Subversion requirement to
configure Slicer, Subversion is still needed to build:
 * the official Slicer installers
 * as well as the extension index (indeed few extensions still host their
 source code using SVN)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@27060 3bd1e089-480b-0410-8dfb-8563597acbee