switching to standard detection mechanism for gtest (may work with 1.7.0 only), refs #1239

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17206 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
removing explicit gtest reference, refs #1239

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17207 29f149b2-0c63-11e1-8f5c-a347f6a9acd0