Allow build script feedback to the crate compiled

Allow build script feedback to the crate compiled

This commit enables the build script for a crate to provide feedback to the
crate itself about how it should be built. This is done through the `--cfg`
flags of the compiler, and each build script is now allowed to print `rustc-cfg`
directives to inform Cargo about what `--cfg` flags it should pass.

All `--cfg` flags are local to the current crate and are not propagated outwards
to transitive dependencies. The primary use-case that this feature is targeting
is compile-time feature detection for applications like C bindings or C
libraries where the version being targeted may change over time.

Closes #1478