Currently GTK can be built with G_ENABLE_DEBUG which enables various debug code and parsing
of those env vars, or without, which instead of parsing them prints a warning if they are set.
While building with G_ENABLE_DEBUG isn't strictly needed it's the only way to make GTK_DEBUG=interactive work,
which is a nice thing to have always.

This enables parsing of those env vars in any case and allows specific values being marked as also
available when not built with G_ENABLE_DEBUG (interactive for example). If not built with G_ENABLE_DEBUG
then all unavailable values will be marked as such in the help output and a note is added that
GTK needs to be built with G_ENABLE_DEBUG to use them, which should help discoverability.