Rewrite build.rs

[mohanson]

1. Build using clang instead of gcc on Windows.
2. The code in build.rs has been significantly simplified.
3. The test worked fine on SP1.

Ref #511

[mohanson]

@eval-exec Do you want this change to be included in the release version?

cc @chenyukang

[eval-exec]

Not sure do we really need to hardcode clang and ignore CC env var here?
cc-rs allow cargo usage like: CC=clang cargo build .....

[mohanson]

Good idea, perhaps we don't need to specify compiler.

[mohanson]

However, if we do this, we will always need to set the CC environment variable when compiling on Windows, see https://github.com/nervosnetwork/ckb-vm/actions/runs/24328961143/job/71030179557?pr=512

[mohanson]

Or should we do it the old way: not specifying a compiler on Unix, but specifying a compiler on Windows?

[eval-exec]

How about create .cargo/Cargo.toml, config CC=clang in [env]?
https://doc.rust-lang.org/cargo/reference/config.html#env

Or should we do it the old way: not specifying a compiler on Unix, but specifying a compiler on Windows?

Both are fine.

[mohanson]

Not a good idea. On Windows, clang is simply called clang, but on Unix, clang is usually named clang-19, clang-20, clang-21, and so on. This is why I initially introduced the clang-finder package.

[eval-exec]

cc-rs support: <var>_<target> - for example:

• CC_x86_64_pc_windows_msvc
• CC_x86_64_unknown_linux_gnu

https://docs.rs/cc/latest/cc/

[eval-exec]

Does ckb-vm have min/max version requirements for clang?

[mohanson]

There are no restrictions on clang versions.