Motivation for this change

Mesa recently stopped installing EGL/eglplatform.h because it is provided by libglvnd. epoxy's headers includes eglplatform.h, and epoxy is in turn required by gtk3. This caused the build of gtk3 to fail on armv7l. The reason it didn't fail on other platforms is that libGL (which propagates libglvnd) is normally propagated by cairo. Back in 2013, GL support was disabled in cairo on 32-bit ARM because apparently mesa didn't build. This is no longer the case, so this PR re-enables GL support in cairo on ARM and MIPS. I don't have a way to test MIPS, and I'm not even sure anyone tests it anymore.

This PR does not fix the root cause of the problem, which is that epoxy.dev should propagate libGL.dev (or maybe libglvnd.dev), but I'll leave that to others if they want to tackle it.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

gtk3: @7c6f434c @vcunat @lethalman @worldofpeace
epoxy: @cillianderoiste
cairo: nobody...