Add CC_PREFER_CLANG_CL_OVER_MSVC environment variable to override prefer_clang_cl_over_msvc

[ArchangelX360]

Fixes #1673

[ArchangelX360]

If I understood correclty, getenv or getenv_boolean won't default to the host environment, so it's not applicable to us here as we actually want to have the host environment leak to us if I'm not mistaken.

However, how can I test that? Because tests are run in parallel, so in my current test implementation it just does not work as tests are leaking that env to one another most probably.

[madsmtm]

See tests/cc_env.rs

[ArchangelX360]

Thanks I'll implement it that way for now.

Then we can discuss the following here: would it be an acceptable design for you to always propagate CC_PREFER_CLANG_CL_OVER_MSVC to self.env in the Build::new constructor?

There is two advantages to that:

• I don't have to use the main function trick in tests and rely on setting self.env
• I can use self.getenv_boolean in the implementation of Build

CC_FORCE_DISABLE would be under the same regime too, for example.

[madsmtm]

I'm not entirely sure I understand the question, but you might wanna wait for #1656 before making too many bold changes to how environment variables work?

[madsmtm]

I think I'd prefer to do things at the usage site instead, something like:

let prefer_clang_cl_over_msvc_env = self.getenv_boolean("CC_PREFER_CLANG_CL_OVER_MSVC");
let msvc = if prefer_clang_cl_over_msvc_env.unwrap_or(self.prefer_clang_cl_over_msvc) {
    "clang-cl.exe"
} else {
    "cl.exe"
};