qdoc silently breaks if the system clang version changes

We found that the generated Qt documentation was around half the size as expected.

Running qdoc with the –debug option showed that it was not able to find system header files.

This is because qdoc was built against clang 6.0.0, but the installed clang got updated to 6.0.1.

qdoc forced clang to use /usr/lib64/clang/6.0.0/include but only /usr/lib64/clang/6.0.1/include exists on that system.

It would also be appreciated if qdoc would bail out with a non-0 exit code if issues like that are detected.