Multiple VCS root, but only one should trigger a build

Is this possible, other than hacking the update interval to 99999999?

I want a build configuration which has the concept of a primary repo, which triggers the build, but also a secondary repo from which I want to grab some files that the primary repo needs to build. These are some cmake files which set up some environment variables and so forth, which the CMakeLists for the primary repo then includes when it configures.

So I've set up two VCS Roots, one for the primary repo that contains the CMakeLists for the build, build script etc and should be triggered as usual, and then a second for the secondary repo which just clones a handful of files into the same directory.

However, if I change any of these files in the secondary repo I don't want a new build to be triggered; I would prefer that this is a manual step, or it can pick them up the next time the primary repo is triggered.

Is there a better/different workflow I should be using to achieve this?