Automate gtest download by Xcode

[maxitg]

The problem

Currently, in order to start using the tests in the Xcode project, one has to manually run cmake to get gtest downloaded. It would be nice to make Xcode download it itself at first build (perhaps by calling cmake in a pre-build script).

Possible solution

Perhaps it could check whether the gtest files exist, and if not, it can just run cmake to build with tests:

mkdir build && cd build
cmake .. -DSET_REPLACE_BUILD_TESTING=ON
cmake --build .