[CMake] One target being built several times during parallel build

[John Drescher]

> Since no one has been motivated enough to address it in the 9 year history
> of the project I wouldn't consider it a showstopping bug.  Besides, even
> if one tries this the build should still complete successfully...just with
> some waste.
>
This has not caused me any trouble on gentoo and also the waste can be
reduced if the user has ccache enabled which IMO should be enabled on
most gentoo boxes since you build the entire os and all software from
source. ccache saves an enormous amount of time installing upgrades
and patches since most of the files will not change.

John