AUTOMOC: processing .hh files can break existing projects
Among the problems reported in #20081 (closed) is that the OpenMesh
OpenMesh-8.0 branch does not build with CMake 3.16.0 (or 3.16.1). I bisected the problem down to commit 4a915453 from !3511 (merged) for #13904 (closed). The problem is caused by handling
.hh files with AUTOMOC where we didn't before.