cmake: fix races without ninja

This fixes build races which happened if "Unix Makefiles" instead of
ninja-build was used as the cmake backend.

For any dependencies of commands on files we need to create a target.
Otherwise, if "Unix Makefiles" are used as the generator the commands
are run in parallel on the different files which often can lead to
races or redundancies in our build.

A nice write-up can be found here:
https://samthursfield.wordpress.com/2015/11/21/
cmake-dependencies-between-targets-and-files-and-custom-commands/#
custom-commands-and-parallel-make