WIP: COMP: Include Halide v18.0.0 in build system

[NicerNewerCar]

@allemangD
Closes #2

A few questions/notes regarding the implementation:

1. Should the logic be moved from the main CMakeList to a GetHalide.cmake (or another name) file?
2. When linking against Halide zlib is required, should we also build/include this in the build system? Or require the user to have this installed already?
3. Everything is working up until I am linking against Halide::Generator, which throws errors since it cannot find llvm, what are your thoughts here?

[ 70%] Built target HalideFilters
[ 74%] Linking CXX executable /root/ITK-build/bin/HalideFiltersTestDriver
/usr/bin/ld: cannot find -lLLVMAnalysis: No such file or directory
/usr/bin/ld: cannot find -lLLVMBitReader: No such file or directory
/usr/bin/ld: cannot find -lLLVMBitWriter: No such file or directory
/usr/bin/ld: cannot find -lLLVMPasses: No such file or directory
/usr/bin/ld: cannot find -lLLVMObject: No such file or directory
/usr/bin/ld: cannot find -lLLVMOrcShared: No such file or directory
/usr/bin/ld: cannot find -lLLVMOrcTargetProcess: No such file or directory
/usr/bin/ld: cannot find -lLLVMSupport: No such file or directory
/usr/bin/ld: cannot find -lLLVMTargetParser: No such file or directory

So far I have tested this on Linux and Windows.

[allemangD]

The issue was two-fold. The ITK filter should not link to Halide::Generator; there must be a generator executable that links to Halide::Generator, and use add_halide_library to actually produce the linkable library and header.

The LLVM linking issues arise because the precompiled Halide static libary does not bundle LLVM, and by default Halide::Generator uses that static library. The precompiled Halide shared library does, as it is used for JIT compilation. So, explicitly setting find_package(... shared) forces the generator to use the shared library and avoid the explicit LLVM dependency.

To test that things link correctly I use the "my_first_generator" example from the tutorial, but I don't do anything with it apart from importing the header.

[allemangD]

I'm not quite sure why the CI runs are failing... I think it may just be that I had not closed the changes in my initial review, so they were not actually re-run? I just approved those changes and manually re-ran the actions to see what happens.

It looks like for some reason the FetchContent logic is not triggered, so find_package(Halide REQUIRED) fails.

https://open.cdash.org/builds/9838219/configure

[allemangD]

CI seems inconsistent; I tested locally with an equivalent dashboard.cmake with no errors, and indeed the latest build-test-cxx ubuntu-22.04 succeeded without issue. I'll go ahead and merge/close this and we can resolve the CI inconsistency separately, in parallel with work to proceed with implementation.

[allemangD]

Normally I would rebase-and-merge, but I'll squash to clean up the "WIP" commits.