coqide 8.10 should depend on gnome-icon-theme (especially on Mac)

[Paolo G. Giarrusso]

After opam install coqide on my Mac (from ocaml/opam-repository#15017, which follows this repo), I had to install gnome-icon-theme for the toolbar to work. That package isn't always installed, and not just on Mac. Seems we'd need to depend on a new conf-gnome-icon-theme package containing:

depexts: [
  ["gnome-icon-theme"] {os = "macos" & os-distribution = "homebrew"}
]

and other such lines. I wrote that following https://github.com/ocaml/opam-repository/blob/master/packages/conf-gnomecanvas/conf-gnomecanvas.2/opam.

[Paolo G. Giarrusso]

And this fix is also needed on opam (see #944); nevertheless, it should be done here to apply to future releases and to dev packages.

[Paolo G. Giarrusso]

Closed by #960 and ocaml/opam-repository#15050.