The most complex problems were from dealing with switches reverted in
the meantime (gcc5, gmp6, ncurses6).
It's likely that darwin is (still) broken nontrivially.
Allowing to use nixpkgs config to provide different defaults is not
going to help us here, so we would like to use nsd.override {} in order
to supply the correct options in the module.
Eventually removing the nixpkgs config option would make sense here as
well.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
