Again, please keep the discussion on the mailing list.
I disagree with your "proper behaviour", since you should never
put the lilypond bin/ directory in your PATH. For help settng up
lilypond, see:
http://lilypond.org/macos-x.html
"macosx on the command line".
I am not opposed to considering a patch which modifies the
ghostscript build in GUB, although I'm not yet convinced that this
is needed.
Hopefully a bug squad member will open an issue for this. That
issue should contain a link to this discussion in the archives,
along with the above warning from me.