Building Qt v5.11.x Documentation Fails on OS X/macOS Under Homebrew

As reported downstream to Homebrew by me in a comment on downstream issue #28455 and responded to by GitHub user 'apjanke' with a reply on that same issue's discussion thread and the creation of Homebrew issue #29415, building Qt's documentation fails when asked for on OS X/macOS during a build of Qt, either version 5.11.0 or 5.11.1, from source using said package manager.

In particular, this is because `qdoc` doesn't get build, apparently because, per further discussion in Homebrew issue #29415, your build system is having trouble finding Xcode's copy of `libclang` (at least under Homebrew, but possibly outside of that context, too.)

Homebrew has thus temporarily or permanently dropped support for building Qt's documentation locally (after failing to fix this issue by making Qt's build system use Homebrew's LLVM's `libclang` instead of Xcode's) as a result.

This is, however, obviously a regression, so it was suggested downstream that one of the individuals involved with this issue's counterpart there ask upstream for guidance on near-future directions with respect to handling it (thus my creation of this issue.)