Mac actions build broken

[cookpa]

Since the addition of extra Mac OS compiler flags in ca26218, the macos action build is failing. I've tried adding the flags to both CMAKE_CXX_FLAGS only (as in the commit before) and both CMAKE_C_FLAGS and CMAKE_CXX_FLAGS.

@stnava would you mind trying out #70? Instead of setting the flags explicitly, I set CMAKE_OSX_SYSROOT, which gives CMAKE the path to the SDK and builds on GHA