Add Windows notes

not obvious, might be helpful
Tweak wording around gcc on windows
Support use of namespaced environment variables based on TARGET and HOST
Merge pull request #9 from jmesmon/namespace-env

Support use of namespaced environment variables based on "TARGET" and "HOST"
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
Fix README link failed

add `cargo's build-script driver` link
Merge pull request #10 from bvssvni/osx

Fix case when 'HOST' is not set on OSX
Merge pull request #11 from SerhoLiu/fix-readme-link

Fix README link failed
Don't fail when all envs are unset. Also log envs checked.
Merge pull request #12 from DiamondLovesYou/cross-compile-fix

Don't fail when all envs are unset. Also log envs checked.
Remove uneeded feature
Merge pull request #13 from sfackler/master

Remove uneeded feature