Release tarballs fail to build due to missing gtest submodule

[dpogue]

GitHub tarballs are generated using git-archive, which does not include the contents of git submodules. In our case, that means the gtest folder is empty.

This causes problems if you try to run cmake with the contents of a downloaded tarball because string_theory defaults to trying to build tests, and fails when it can't find gtest.

Our options appear to be going back to vendoring gtest in the repo (rather than as a submodule) or adding some detection of the gtest folder before trying to enable tests.