Support finding Windows tools on non-Windows host

[cavivie]

This PR contains a simple implementation of building targets on non-Windows platform. it supports manually set environment variables to find Windows msvc tools.

By setting VCINSTALLDIR environment variable, we can search msvc tools simply, and resolve related issues:

• Failed to cross compile rust-openssl(openssl-src-rs) for windows msvc targets via cross-rs #797

Currently, this PR can be regarded as a draft implementation, and the implementation details may still be discussed.

[cavivie]

@ChrisDenton It would be great if you could give me some advice.

[NobodyXu]

cc @thomcc I will need your help on how to move forward with this PR

[maaft]

subbing. very interested to be able to use cross for msvc compilation.

[NobodyXu]

Going through this again, I think it's ok to merge in this PR.
At the very least it shouldn't regress anything, then it will be perfectly fine for it to merge in.

If we find any issue with it lately, we could just fix it and cut a new release.

[NobodyXu]

cc @cavivie Can you rebase this please?

I'm willing to merge it in.

[cavivie]

cc @cavivie Can you rebase this please?

I'm willing to merge it in.

Okay, I will do it as soon as possible.

[NobodyXu]

Thank you!