Fix case when 'HOST' is not set on OSX
Fix case when 'HOST' is not set on OSX

Instead of using “gcc” it fails, so by using `Result` instead of
`Option` the error message can be returned instead of causing task
panic.

Fixes https://github.com/rust-lang/time/issues/28

alexcrichton/gcc-rs,kosta/gcc-rs,alexcrichton/gcc-rs,kosta/gcc-rs,kosta/gcc-rs,gabrielesvelto/gcc-rs,alexcrichton/gcc-rs,gabrielesvelto/gcc-rs,gabrielesvelto/gcc-rs