Gtest is now only built if the testing tree is built

Unfortunately I am not able to setup a full windows build environment until the next release, so I cannot fix the root cause of gtest failing with mingw. 

The reason why gtest was compiled even if `BUILD_TESTING` was set to off is that find_package calls  `add_subdirectory` on the gtest folder. As far as I know this is done in order to include all variables etc. defined in the gtest CMakeList. I am not quite sure about the difference between `ENABLE_TESTING` and `BUILD_TESTING`, therefore this fix may introduce other issues. But at least it should fix the failing build described in #131