Remove SVN_REVISION preprocessor definition.

Remove some stuff from the CMakeLists.txt that looked for subversion. We could potentially try to replace that definition with a GIT_SHA1 definition.

The SVN_REVISION flag did not seem to be used.

 http://stackoverflow.com/questions/1435953/how-can-i-pass-git-sha1-to-compiler-as-definition-using-cmake