[CMake] Force target to always run last?

[Robert Dailey]

I have a custom target that must meet the following requirements:

* It must always run, regardless of what subset of other targets are being built
* It must always be the very last thing run. In parallelized builds,
it must wait until all other targets are done building before
starting, so that it is the very last target run, and should not run
in parallel  with others.

Is this possible? I'm willing to use hackery if needed...

Running CMake 3.8.0. Thanks!