If I try to force it to build (mkcd ~/dev/qt5-dbg/qttools/src/qdoc; qmake ~/dev/qt5/qttools/src/qdoc; make) I get

/home/rutledge/dev/qt5/qttools/src/qdoc/clangcodeparser.cpp:59:10: fatal error: clang-c/Index.h: No such file or directory
 #include <clang-c/Index.h>
          ^~~~~~~~~~~~~~~~~

/usr/lib/llvm-6.0/include/clang-c/Index.h exists, but probably needs to be added to the include path.  (Of course maybe it's an Ubuntu bug that there's no symlink?)  I have libclang-dev installed.

If you google you can find others complaining about other packages faililng because /usr/include/clang-c/Index.h is not found.