Document LLVM dependency of qdoc

From Qt 5.11 onwards, qdoc uses libclang (version >= 3.9.0).

This should arguably be documented in

http://doc.qt.io/qt-5/build-sources.html

Windows

You can use the precompiled "Clang for Windows" binaries from http://releases.llvm.org/download.html:

• Download latest version of either 'Clang for Windows (32 bit)' or 'Clang for Windows (64 bit)', depending on your target architecture
• Run the downloaded installer. The default installation directory is "C:\Program Files\LLVM"
• Prepare build environment (before running configure/make)
  • set LLVM_INSTALL_DIR=C:\Program Files\LLVM
  • set PATH=%PATH%;%LLVM_INSTALL_DIR%\bin

• Continue configure as normal. Note that, because a clang compiler is now in PATH, you might need to explicitly pass a platform argument to configure:
  • configure -platform win32-g++ ...

Linux

Install libclang and the matching development packages from your package system. Note that qdoc requires a somewhat recent version (4.9.0), so possibly you have to build and install libclang yourself.

Qt tries to find llvm-config in /usr/bin. If it is located elsewhere you have to specify the top directory (without bin) by setting the LLVM_INSTALL_DIR environment variable:

• export LLVM_INSTALL_DIR=/usr/local

Mac

Qt looks for homebrew llvm by default

• brew install llvm
• Set LLVM_INSTALL_DIR if you have a non-standard homebrew location (or want to use another llvm install)