Add macOS 10.15 build to CI

[skissane]

Note this build workflow lies a bit - it tells CMake we are building with GCC (-DTOOLCHAIN=gcc), but actually on macOS by default gcc is a wrapper for AppleClang. I did it this way because it was the simplest way to get it to work. Passing -DTOOLCHAIN=clang fails with a bunch of errors due to clang not liking the parameters that CMake is passing.