Embedding of gtest/gmock in source tree

The current approach used by yaml-cpp is to embed gtest/gmock in `test/gmock-1.7.0`.  While this works fine for the most part, it doesn't work well when you're integrating it into a larger build which also includes its own gtest, e.g. a later version such as 1.8.0.  It would be nice if this could be made optional.  The headers and libraries can conflict.



As a suggestion, this is the approach I took in a different project: [GTest.cmake](https://github.com/ome/ome-files-cpp/blob/v0.2.0/cmake/GTest.cmake).  Here, we pass in a variable `GTEST_SOURCE` which points to the location of the sources.  If unset, we simply use `find_package` and use an external version.  If set, we use the internal version (or the version pointed to outside the source tree).  An option could be added to yaml-cpp to do an equivalent action, e.g. `build-gtest` or `build-gmock`.  It could even be defaulted based upon the result of `find_package`.



Also worth noting: gtest 1.8 combines gmock and no longer enforces the requirement to "vendor" into a source tree.  It's now locally installable again, with the proviso that it needs building with a compatible set of compiler options (so no different than most other libraries!).





Regards,

Roger

[]