Lift the requirement that cargo is built with make

Cargo should be able to build with cargo so others can depend on the cargo
library. This means that the one requirement of the CFG_VERSION env var will not
be available. This alters `version()` to prioritize CFG_VERSION but fall back to
the cargo-specified variables.
auto merge of #328 : alexcrichton/cargo/cargo-building-cargo, r=wycats

Cargo should be able to build with cargo so others can depend on the cargo
library. This means that the one requirement of the CFG_VERSION env var will not
be available. This alters `version()` to prioritize CFG_VERSION but fall back to
the cargo-specified variables.