Background: graphics/gnuplot builds with

     USE_GNU_READLINE=     YES

and

     CONFIGURE_ARGS+=      --with-readline=${BUILDLINK_PREFIX.readline}

I prefer gnuplot's (limited) internal readline because 'history' works
with it but not with GNU readline. Every time gnuplot gets rebuilt, I
forget that I need to change those settings, so end up building and
installing again. I'd like to make appropriate settings in a central place
(mk.conf).

Is there a general mechanism for adjusting package-specific make
variables? I've been wading through the various *.mk files trying to
become more familiar with pkgsrc, but right now I can't see the forest....

Thanks,
Mark