Generalising CMake

[LiamOS]

While trying to build duneanaobj with spack I noticed that it completed rather easily, later realising that the CMake files effectively depend on UPS/CET and do something between nothing and something useful when not found.

I've made some changes to the CMake files to build and install correctly. I haven't validated this fully, more changes may come if/when I find something I missed.

[chenel]

I think we will need to wait until @tomjunk comes back from Fermilab shutdown to review this---I'm completely unclear on exactly what's good or not good from a Spack perspective.

[LiamOS]

Spack is pretty flexible; as long as the build system is configurable in a somewhat normal way, the spack packaging can handle any quirks. The main expectation is that it build and install in a somewhat similar way to how most software would be built with CMake, which this would not if it could not find UPS stuff.

I think the changes I've made shouldn't break the SL7 builds, but would need someone to validate. Although this may become a non-issue when (...if?) spack becomes our everyday driver.

There's also no rush on merging this, for testing I'm telling spack to pull from this branch rather than main.