Observe CARGO_CFG_TARGET_* in build.rs

[briansmith]

In particular, the CARGO_CFG_TARGET_FEATURE variable should be used to help with the CRT stuff on Windows. Note "After this change, the gcc-rs crate will be modified to check for the CARGO_CFG_TARGET_FEATURE directive, and parse it into a list of enabled features."

These look like they'd be useful in our build.rs:

export CARGO_CFG_TARGET_OS=linux
export CARGO_CFG_TARGET_FAMILY=unix
export CARGO_CFG_TARGET_ARCH=x86_64
export CARGO_CFG_TARGET_ENDIAN=little
export CARGO_CFG_TARGET_POINTER_WIDTH=64
export CARGO_CFG_TARGET_ENV=gnu
export CARGO_CFG_UNIX
export CARGO_CFG_DEBUG_ASSERTIONS
export CARGO_CFG_TARGET_FEATURE=sse,crt-static

In particular, if we add the dependency on the target_build_utils crate in #321, then we might be able to remove that dependency by using these variables.

[briansmith]

This depends on rust-lang/rust#37406.

[briansmith]

It seems like Cargo is passing in these variables and we're able to use them. This was done in 5c77732.