auto merge of #277 : alexcrichton/cargo/add-configure-script, r=wycats

This configure script is similar to rust's in that it doesn't require anything
like autotools, it's just a meta-script to "generate" a makefile and perform
run-of-the-mill validation/discovery before the Makefile is run.

The main purpose of this rewrite is to support multi-target and targeted builds.
This will allow us to produce 32-bit snapshots for platforms as well as easily
providing `./configure --target $foo` for initialization.

cc #274