Check support for macOS

In principle we wish to support both linux and macOS. I have attempted to compile on my macOS (Catalina Version 10.15.6) and there are issues in the main src/CMakeLists.txt file, specifically in how it checks whether you are on a macOS machine:

CMake Warning at src/CMakeLists.txt:12 (message):
  You are using an unsupported compiler! Compilation has only been tested
  with Clang and GCC.

I believe the check for Clang here needs to be updated.

I have not yet checked where we have explicit dependencies on linux (e.g. specific linux headers being included).