Summary: Pull Request resolved: https://github.com/facebook/yoga/pull/1334 Last change made me discover CMake on Ubuntu defaults to GCC. This change makes it so that we explicitly configure whether to use Clang/LLVM or GCC when building on machines with the `ubuntu-latest` image. Reviewed By: christophpurrer Differential Revision: D47462132 fbshipit-source-id: e81fb6d4b1740ab2913d39b6e90c52d674e5a4f2