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:

• https://github.com/rust-lang/cc-rs/issues/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]

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.