Update options with meson setup <builddir> -Dopt=value

If the build directory already exists, options are updated with their new value given on the command line (-Dopt=value). Unless --reconfigure is also specified, this won't reconfigure immediately. This has the same behaviour as meson configure <builddir> -Dopt=value.

Previous Meson versions were simply a no-op.

Clear persistent cache with meson setup --clearcache

Just like meson configure --clearcache, it is now possible to clear the cache and reconfigure in a single command with meson setup --clearcache --reconfigure <builddir>.