Status update.

* configure works: it checks options (like install does) and writes to a file.

* New design for build: It uses set_undefined_options to get options from configure (previously it read the cache file, but this caused nasty issues; I’ll document those designs some day, it was instructional for me).  All tests pass.

* Same thing for install, but it’s harder to make it work.  install.finalize_options checks for conflicts (for example you can’t set --user and --prefix=x) and then fills all options (so prefix gets a value).  Now configure has the same checks, so that it does not write non-working options to the cache file.  The problem is that all options are set in configure.finalize_install_options and then passed to install with set_undefined_options, and then install.finalize_options shouts at you because you can’t set all options.  I tried a simple and stupid fix for that (disable option conflict check when options come from configure), which fixed all configure tests but caused test_core to fail.  This means that while the configure tests passed, they were only superficially working; I have to be suspicious to my own tests and out general test coverage.