New module: build environment requirements

[zml2008]

This probably makes most sense as a Settings plugin:

• make assertions about the build environment
• provide friendly error messages
• be maximally compatible (back to whatever ancient version of Gradle 4 is present in some debian repos)
• allow downgrading the assertions to warnings
• provide assertions about:
  • java runtime version (range handling)
  • java runtime vendor
  • operating system?
  • system architecture
  • gradle version
  • git submodule existence
  • git submodule up-to-date
  • executable on PATH