Small CMake related tweaks

[cpu]

add doc header to CMakeLists.txt

At least one person convinced themselves CMake is the required/preferred way to build librustls despite the README documenting the opposite. Let's try to be even more overt about it by adding a header to the root CMakeLists.txt.

In the near future I'd like to lift all of the CMake + test C code into a separate crate for even clearer separation.

rm CI dep on rustls-ffi-test for artifact test

Previously the artifacts.yaml CI job relied on an external repo that had a simple C/CMake application depending on librustls. When verifying artifacts we cloned this repo and tested a build against the artifact.

This commit reworks the main repo's test application code/build to support this use-case. The main tweak required is to support not building librustls from src, but instead finding it with pkg-config. We gate this behind a new FORCE_SYSTEM_RUSTLS option. The artifact test CI is updated to use this option. We want to leave the old default because it's the most convenient for developers of librustls: you want to hack on the Rust code and have your C test code builds pick up the changes without extra ceremony.

Annoyingly for the pkg-config workflow with the release artifacts zips (not the deb) we have to patch the .pc prefix to an absolute dir to get this working nicely, which means handling Windows specially.

Similarly, for yet to be determined reasons, on Win32 we have to add some extra target_link_libraries that don't propagate automatically through the .pc config (would be nice to sort this out one day). For now we just shift the existing Win32 specific target_link_libraries invocation up and use it for both pkg-config librustls and build-it-ourselves librustls.

Resolves #519

abort on CMake install w/ clear error

This updates the librustls CMakeLists.txt to abort on install. It's only intended to be used for the C examples, which we don't want anyone to install anywhere!

Use cargo capi for librustls, not cmake.

$ cmake --install build
-- Install configuration: "Release"
CMake Error at build/cmake_install.cmake:46 (message):
  librustls installation via CMake is not supported.  Use 'cargo capi
  install' instead.

  See:
  https://github.com/rustls/rustls-ffi?tab=readme-ov-file#build-rustls-ffi

Resolves #526

[cpu]

cpu marked this pull request as draft now

I had removed the main CI yaml while iterating on this but it looks like I broke the normal CI workflow's build on Windows with the target_link_libraries change. I'll fix this shortly. Folks can hold review until then if desired.

[cpu]

looks like I broke the normal CI workflow's build on Windows with the target_link_libraries change. I'll fix this shortly.

All fixed. Just needed a small adjustment for consistent target_link_options form between two call-sites on the Win32 path.

@ctz If you could spare time for a review it would be appreciated (no big rush). Thanks!