Update deprecated cmake version

[mazunki]

As discussed in #2310, there are some issues with the build system picking a linker for unikernels, even in --pure mode.

Passing set(CMAKE_LINKER_TYPE LLD) seems to fix this, but doesn't work in older versions of CMake. This PR updates the minimum version of cmake we depend on to 3.31.6 (as reported by nix-shell --pure ./develop.nix --run 'cmake --version').

Bumping this version reports two usages of PRE_BUILD which is invalid in modern CMake (I believe they always have been, but just been silently ignored? https://cmake.org/cmake/help/latest/command/add_custom_command.html), and only be relevant to the Visual Studio editor and build system. I've simply removed them (commit 7097eda).

Lastly, I have added an explicit dependency to LLD/LLVM (https://cmake.org/cmake/help/latest/variable/CMAKE_LINKER_TYPE.html) in the example unikernel. With CMake updated, the unikernel links successfully even on Gentoo .

[mazunki]

Worth nothing: It only links successfully in --pure mode on my machine.

[mazunki]

Seems like set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fuse-ld=lld") from cmake/linux.service.cmake:175 is the culprit for adding the unrecognized -fuse-ld=lld flag. Is this still required if we explicitly tell CMake to use the LLD linker?

Sidenote: Maybe we should enable LLD by default on the linux target since we're using LLVM anyway?

[mazunki]

@alfreb You mentioned you were skeptical to bumping to such a high version because of cmake_minimum_required suggesting it's the minimum version, and not the expected version. Do you prefer if we only bump the outdated ones to 3.10?

Since we're using a pinned nix version, we can essentially guarantee that the version used here is available. Are there any downsides other than a semantic meaning?

For reference, 4.2.0 is the latest version, 3.31.6 was released on 2025-02-24.

[alfreb]

This is fine. I generally don't want to deprecate tools that work just fine, and are supported. But the only build system we support is nix with the pinned versions, and I verified now that this is 3.31.6, so let's go with that, even though it's not technically the minimum version.