Git invocations without an available gitconfig (ex. a test environment)
will fail whereas patch should always work. This adds a build dependecy on patch.

Currently MKL-DNN isn't supported on this platform

This is temporary and will be removed once upstreamed

This was removed because the previous eigen CMake spec forced a new CMake version
but that was taken out in a previous commit

add eigen as external project.
add very simple eigen unit test to test building/linking