I hope this is what you had in mind:

--enable-debug adds -g (unconditionally)
--disable-debug removes -g (if it was already in there somehow)
(giving neither does nothing)

Since none of the templates default CFLAGS with a -g you're not likely
to
end up with two -g flags. Not that they'd hurt though.

It doesn't do anything about C++.

Peter Eisentraut