Supporting new compilers without dropping support for old ones

[KodrAus]

This issue is about using a build script to switch on compiler versions, rather than about a policy for managing minimum compiler versions. See #123 for that.

Cargo build scripts make it possible for a stable library to detect and automatically utilize newer compiler features when they're available in a consuming crate without dropping support for older versions, or requiring dependents to explicitly opt-in. Based on rustc_version's dependents this seems like a pretty common pattern. Some specific examples (that don't all use rustc_version) are:

• serde which uses a build script to provide a convenient macro to Serializers and Deserializers to support 128bit numbers when available and other things.
• regex which uses a build script to determine whether stable SIMD intrinsics are available.
• lazy_static which uses a build script to switch on the internal implementation of Lazy depending on the compiler version.

Each of the libraries above takes a bit of a different approach to detecting specific compilers and threading that information through the library itself. Are there any pitfalls or gotchas with this pattern we could call out or implementation approaches we could recommend?

[KodrAus]

I've converted this into a general discussion because it doesn't really have any bearing on public API.