Taken from a system-wide install of Mono 6.4 on macOS, provided by @neikeq.
This effectively means that we can't use the current Dockerfile.osx to build mono < 6.0, as the DLL path changed.
* Mono config on MacOSX is not portable from the Linux and Windows version. * Rename the compilers so that 'gcc' and 'g++' are the new version by default. * Add default environment variables to containers.