From 5.11, qdoc uses the clang parser and the clang-c API for parsing C++ code. This means the correct libclang must be installed in LLVM_INSTALL_DIR which must be set by the user. If qdoc is to be built from the sources, <clang-c/Index.h> must also be accessible below LLVM_INSTALL_DIR. qdoc requires clang from at least version 3.9 of LLVM.

I don't know how much of this should be checked by the configure script.