libgeos#644 Can't build using cmake with tar ball

only do svn check if there is a .svn file in 
source folder to ensure it works with tar ball

git-svn-id: http://svn.osgeo.org/geos/trunk@3889 5242fede-7e19-0410-aef8-94bd7d2200fb

CMakeLists.txt

@@ -254,7 +254,9 @@ MACRO (GET_SVN_REVISION)
 ENDMACRO(GET_SVN_REVISION)
 
 # Determine SVN/Git revision
-GET_SVN_REVISION()
+if(EXISTS "${CMAKE_SOURCE_DIR}/.svn")
+  GET_SVN_REVISION()
+endif()
 if ( NOT ${Project_WC_REVISION} EQUAL 0 )
    set( GEOS_SVN_REVISION ${Project_WC_REVISION} )
    configure_file (