Emacs darwin: avoid X unless explictly requested

This keeps the dependencies lighter. Perhaps future work on this
derivation could enable the Cocoa interface by default on Mac.
gnuplot darwin: optional gnuplot_aquaterm variant

This variant uses the more Mac-friendly aqua driver, but it requires
that you separately install the AquaTerm package.

Note that AquaTerm is open source and could perhaps be later included
as a nix derivation. If that happens, it would be nice to remove the
gnuplot_aquaterm top-level attribute and just make it the default.
libsoup: update from 2.44.1 to 2.44.2

Enable introspection
Renamed from libsoup_2_40 to libsoup_2_44
gnuplot: pass aquaterm request explicitely (see #1197)
add webkitgtk 2.2.3

With minor changes from @vcunat, see #1197.
gtk3: bugfix update 3.10.4 -> .5
merge #1197: gnuplot darwin changes
mesa: add myself as a maintainer

When I authored much of the current expression,
I should better watch it, too.