libvpx: fix cross-compilation

[Mindavi]

This is done by removing some warnings that are not supported by the
cross-compiler.

We also need to disable the --enable-external-build flag, which breaks
the build completely.
This PR: #100210 added the flag,
but it doesn't explain why it was added. It also doesn't show any
attempt at trying to cross-compile for either darwin or aarch64. So
let's remove it again, since it 'just' seems to break the build anyway.

Motivation for this change

Support cross-compilation for libvpx, which is a dependency of ffmpeg.

I've tested building this with nix build .#pkgsCross.aarch64-multiplatform.libvpx, which works. With #123915 applied, ffmpeg now also cross-compiles for aarch64-multipllatform :).

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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

This reverts a change made in #118783. @TredwellGit do you have any idea why the --enable-external-build flag was added there? It doesn't seem like you attempted a cross-build (looking at the PR at least), but maybe you did?

[TredwellGit]

I added it because 1859b5a did. Please remove it along with the other commented out flags.

[Mindavi]

Thanks for the review. I've updated the PR to remove the commented out flags and to be more specific on which files need patchShebangs. Let me know if there's anything I can improve.

[jonringer]

Please follow CONTRIBUTING.md and manual#submitting-changes-making-patches and squash the fix-up commits.

git rebase -i is a powerful command which achieves this, I created a small video demonstrating it's use here. A more indepth text tutorial can be found here

[Mindavi]

Squashed the commits together. I thought the commits I made were separate enough to warrant a commit each, but this is ok for me too, so let's do it.

[jonringer]

diff LGTM

still need to test

[Mindavi]

Note that libvpx_1_8 still doesn't cross-compile, but only firefox depends on it, so I think it's less of a priority.

That's also the reason why I didn't remove the --enable-external-build flag, it doesn't help anyway.

[SuperSandro2000]

nix-build -A pkgsCross.aarch64-multiplatform.libvpx build for me.