Created attachment 430546 [details, diff]
proposed IUSE=win32 to gtk-2.24.30.ebuild
Currently the gtk+:2.0 ebuilds only has support for aqua, or for X (which is effectively not-aqua). In order for gtk+ to be cross-compiled for windows via mingw, I had to force USE="aqua" and then inject via EXTRA_ECONF the flags to ./configure to disable quartz support and enable win32 support.
This additionally required me to hack x11-libs/cairo in a similar fashion, due to gtk+ depending on cairo[aqua?], despite cairo[-aqua,-X] doing the right thing.
It would be much nicer going forward if the gtk+:2 (and likely :3) ebuilds could have a win32 use flag.
The attached patch is a possible way it could look, note I haven't tested it but the changes are fairly simple. If the idea is friendly I'll report back with test results.
I have no problems with including this... if tests go ok, please feel free to commit
You have my go to apply this to everywhere it fits.