The custom command de-duplication added by commit v3.12.0-rc1~171^2 (VS:
Generate a custom command only in the least dependent target,
2018-03-23) accidentally also applied to the `CMakeLists.txt` file
reference we put in each target.  This file reference comes with a
custom command that has no dependencies and that is safe to run
repeatedly across multiple targets (via internal stamp checking).
Therefore it should be excluded from the de-duplication so that
`CMakeLists.txt` references appear in all targets for human reference.

Fixes: #18310