Installing Clang for QDoc

If you are going to build QDoc from source, the following prerequisites apply:

1. Install LLVM 6.0 or greater. The pre-built binaries needed can be found here.
2. Set the environment variable LLVM_INSTALL_DIR to point to the directory where LLVM is installed. This should be the top level directory. For example, on a Linux or macOS system with LLVM installed to /usr/llvm:

   $ export LLVM_INSTALL_DIR=/usr/llvm

   On a Windows system with LLVM installed to C:\Program Files\LLVM:

   C:\> setx LLVM_INSTALL_DIR "C:\Program Files\LLVM"