"fatal error: 'clang-c/Index.h' file not found" when building qdoc in an existing Qt build

I am yet to migrate all of my build scripts to export LLVM_INSTALL_DIR. When I run a build script where it's missing, qdoc isn't built, so I have to build it manually. I export the variable, create qttools/src/qdoc in my shadow build directory, cd into that directory, run qmake and then make. I get the following error:

Mitchs-MBP:qdoc mitch$ make
compiling /Users/mitch/dev/qt5-dev/qttools/src/qdoc/atom.cpp
compiling /Users/mitch/dev/qt5-dev/qttools/src/qdoc/clangcodeparser.cpp
/Users/mitch/dev/qt5-dev/qttools/src/qdoc/clangcodeparser.cpp:57:10: fatal error: 'clang-c/Index.h'
      file not found
#include <clang-c/Index.h>
         ^~~~~~~~~~~~~~~~~
1 error generated.
make: *** [.obj/clangcodeparser.o] Error 1

The file exists on my system.

I tried e.g. running configure -redo, but it doesn’t help.

I'm able to follow the same procedure with other modules without issues.