FindDoxygen: Disable Doxygen if executable returns non-zero result

This makes detection of Doxygen more robust in case of broken Doxygen
executables. Doxygen is called to determine its version number. If this
call returns a non-zero result, we have an indication that the Doxygen
executable is broken. We report Doxygen as not found to prevent issues
with checks for other Doxygen component, that might lead to
configuration aborts.

Fixes: #18708