MSVC: Don't use environment variables to find tools if the targets differ

[ChrisDenton]

On Windows, when using a "Developer" powershell or command prompt, the "VCINSTALLDIR" environment variable is set. Currently if this is detected by cc-rs, then it will unconditionally assume the environment has already been correctly set up.

This PR adds a check to make sure the Visual Studio target matches the requested Rust target. If not then it falls back to using automatic detection.

Fixes: rust-lang/rust#43468

[ChrisDenton]

After feedback, I've converted this to a draft until I have a moment to update it.

There are concerns with my current approach and how it relies on an environment variable that may not be set for older versions of Visual Studio.

[ChrisDenton]

This PR now adds a find_msvc_environment function that's intended to be used the same way as the other find_msvc_* probe functions. It currently only makes a difference with VS 2017 and 2019 but will fallback to the old behaviour on older versions of Visual Studio. A future PR could expand the function to better support more versions.

This PR shouldn't impact situations where cc was already working properly, but it will help in some cases where it used to give libs for the wrong arch.

[alexcrichton]

Seems reasonable to me!

[alexcrichton]

Thanks for this!