Auto merge of #2910 - BusyJay:master, r=alexcrichton

Fix python detection

Some platform, like ArchLinux, may not have python in the PATH, but can still have python2. So we should add some fallback option to detect python correctly. This detection order is copy from [rust-lang/rust](https://github.com/rust-lang/rust/blob/master/configure#L742).

diff --cc configure
Simple merge