Fix Qt 5.15 on Darwin, make it default

[prusnak]

Motivation for this change

• Fix builds of Qt 5.14 and Qt 5.15 on Darwin by using xcbuild qt514.qtbase build broken on darwin #95199
• Make Qt 5.15 default on Darwin

I tried building qt512.qtbase, qt514.qtbase and qt515.qtbase on x86_64-darwin and it built successfully.

Coming up next: qt512.full, qt514.full and qt515.full ...

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/)
• Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.

[prusnak]

Hm, so qt515.full (more particularly qt515.qtwebengine) requires macOS 10.14 SDK (for Metal backend) after all.

Switching to Draft.

../../3rdparty/chromium/third_party/skia/src/gpu/mtl/GrMtlUtil.h:23:2: error: Must use at least 10.14 SDK to build Metal backend for MacOS
#error Must use at least 10.14 SDK to build Metal backend for MacOS
In file included from ../../3rdparty/chromium/third_party/skia/src/gpu/mtl/GrMtlGpu.mm:8:
In file included from ../../3rdparty/chromium/third_party/skia/src/gpu/mtl/GrMtlGpu.h:20:
../../3rdparty/chromium/third_party/skia/src/gpu/mtl/GrMtlCommandBuffer.h:47:31: error: no type or protocol named 'MTLEvent'
    void encodeSignalEvent(id<MTLEvent>, uint64_t value) SK_API_AVAILABLE(macos(10.14), ios(12.0));

[prusnak]

Closing this, superseded by #125572 which cherry-picks the first two commits.

[smancill]

Can a new issue be created, because this comment is misleading, and it's hard to track that even with #95199 closed it's still not ready:

  # TODO bump to 5.14 on darwin once it's not broken; see #95199