[machdep] more robust generator

- --compile-flags="-c -I/path" works (i.e. there's no need to provide as many
  --compile-flags options as you have flags to pass to the compiler itself.
  Ditto for --cpp-arch-flags of course
- we start by checking that a minimal C file does not trigger a compiler error
  with the given option and we abort otherwise. This ensures that subsequent
  tests will not mistakenly take a configuration error for a genuine result.