MacOS: check suitability of XCode

[MSoegtropIMC]

Some packages (gmp-ecm, gappa) require full XCode - not the command line version apparently because they use gm4 which comes eith full XCode but not the command line versions.

If full XCode is installed, it can be that it is installed, but the license is not accepted which makes OCaml configure fail.

We should auto check if a suitable XCode is installed and the license has been accepted.

See https://stackoverflow.com/questions/58632629/how-to-check-if-xcode-license-needs-to-be-accepted