[CMake] Make cmake config more reliable?

[maerhart]

I've heard from someone who ported CIRCT main to work with mlir-16 that the LLVM cmake checks complain that there are cpp files not included in any library. I'm not sure if that's caused by changes in the LLVM cmake config since then or if generally all cpp files in that directory should be marked optional when there are multiple libraries declared in a CMakeLists.txt.

Could someone with more CMake knowledge take a look if that change makes sense?

[darthscsi]

This looks ok to me, but I don't know the CMake issues involved. If this doesn't change the build, I don't see a reason not to make this change.

[darthscsi]

I don't see a reason not to do this.

Though I don't know what the optional sources are for in the first place.

[maerhart]

Thanks @darthscsi for taking a look!
I think the optional sources are just here to tell the LLVM CMake checkers that it shouldn't check that the file is used somewhere. Apparently this is always needed when there are multiple libraries defined in one CMakeLists file.