cmake: libclang could not be located

$ cd ~/dev/qt6-dbg-clang (an empty directory)
$ cmake ../qt6 -DBUILD_WITH_PCH=OFF -DBUILD_TESTING=ON -GNinja
 ...

WARNING: QDoc will not be compiled, probably because libclang could not be located. This means that you cannot build the Qt documentation.
 Either ensure that llvm-config is in your PATH environment variable, or set LLVM_INSTALL_DIR to the location of your llvm installation. On Linux systems, you may be able to install libclang by installing the libclang-dev or libclang-devel package, depending on your distribution. On macOS, you can use Homebrew's llvm package. On Windows, you must set LLVM_INSTALL_DIR to the installation path.
 WARNING: Clang-based lupdate parser will not be available. LLVM and Clang C++ libraries have not been found.

Qt is now configured for building. Just run 'cmake --build . --parallel'
 Once everything is built, you must run 'cmake --install .'
 Qt will be installed into '/usr/local'
 To configure and build other modules, you can use the following convenience script:
 /usr/local/bin/qt-cmake

If reconfiguration fails for some reason, try to remove 'CMakeCache.txt' from the build directory

– Configuring done
 – Generating done
 – Build files have been written to: /home/rutledge/dev/qt6-dbg-clang
 $ locate libclang
 ...
 /usr/lib/libclang-cpp.so
 /usr/lib/libclang-cpp.so.10
 /usr/lib/libclang.so
 /usr/lib/libclang.so.10
 ...