Add --enable|disable-cxxmodules option to configure.

Remove the ugly cxxmodule workarounds in graf3d: they don't really work anyway.
Instead, explicitly disable building with cxxmodules graf3d on macos due to
glew. Disable building with cxxmodules libs which still have bugs.

Improve the build system implementation scaffolding for cxxmodules.

Finally, with the introduced infrastructure and workarounds, ROOT builds and
runs with cxxmodules on (given one has newest clang, with my pending for
mainline patch). It seems that roottest is almost happy, too.