This includes two layered changes so the gtk2hs packages build on Darwin:
- For `glib`, `gio`, `gtk`, `gtk3`, and `pango`: the fix for version 0.13.8.0
from https://github.com/gtk2hs/gtk2hs/pull/293 . I expect at some point the
referenced fix (or one like it) will be released and and brought into
nixpkgs, at which point the override and patch files here can (in fact must)
be removed.
- For `gtk` and `gtk3`: also apply the required cabal flag cited in
https://github.com/gtk2hs/gtk2hs/issues/249 to specify the Quartz rather than
X11 backend (Quartz is the one that both nixpkgs and macOS support
out-the-box). This override is likely to be wanted indefinitely.
Both modifications are required for a successful build of `gtk` or `gtk3` on
Darwin right now.
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