Add new feature to allow building ckb-std using clang as C compiler

[xxuejie]

No description provided.

[blckngm]

The cc crate allows setting the compiler via the CC or CC_<target> environment variables. Why not support clang with these and https://docs.rs/cc/latest/cc/struct.Tool.html#method.is_like_clang.

[xxuejie]

In fact, I did use environment variables including CC, CC_<target> or TARGET_CC to set the compiler to use a lot in the past. I have mixed feelings here: overriding CC can be dangerous since this is also picked up by most C compilers, when the project is rather sophisticated, overriding CC can be quite nasty. For example a secp256k1 dependency would have native C compiler compiling native binary to generate certain constant tables, and one might need to use an external script to make sure the C project is compiled without overriding CC, and only set CC in cargo build.

CC_<target> and TARGET_CC has a nicer story here since they are specific to Rust. But still one needs to remember to set the environment variables in advance.

Having a feature we can use here, IMHO, is actually simpler to some minds, since all you need to do is a simple cargo build, there is no need to setup the environment somehow.

But I do agree it does provide values to support changing compilers via CC, CC_<target> or TARGET_CC, and it makes sense to infer the actual compiler via Tool::is_like_clang, I've prepared a PR for this change here