RLS and cargo check run sometimes unnecesary build.rs steps

[robmv]

When running an IDE using RLS or just running cargo check, some large projects using crates that have big static builds of non Rust based native libraries, the build.rs of those dependencies run a complete build of the non Rust code.

One example is openssl, when using it in vendored mode, takes a lot of time to get feedback on an IDE because it will build openssl entirely, something it isn't needed for getting the information RLS or cargo check needs, they don't need a complete build of those crates.

Maybe standardizing some kind of environment variable passed to the build.rs program could tell these dependencies that the build is being run for RLS or cargo check, and these scripts could be updated to be smarter, avoiding the native build in these circumstances.

This will require cooperation of build.rs users but it is better than than the current status where an IDE takes too much time to give some feedback.

[nox]

This is also a big issue for Servo, where we have to build SpiderMonkey, osmesa, skia, angle, and a bunch of other C++ projects, which takes a lot of time.

I worked around the issue for RLS by checking that $CARGO --version starts with "rls ", but I can't do that for cargo check.

[jyn514]

I think this is the same as #4001.

[ehuss]

Yea, I'm going to close as a duplicate of #4001.