rustc's target triples generally only have a vague resemblance to each
other and to the information needed by `cc`. Let's instead prefer
`CARGO_CFG_*` variables when available, since these contain the
information directly from the compiler itself.

In the cases where it isn't available (i.e. when running outside of a
build script), we fall back to parsing the target triple, but instead of
doing it in an ad-hoc fashion with string manipulation, we do it in a
more structured fashion up front.