More robust support for conditional compilation

[brson]

Use item attributes to support conditional compilation. Items marked with a certain attribute, e.g. #[cfg(test)] will only be translated when rustc is passed a flag for the 'test' configuration.

Remove the target_os hack (as used in std.rc)

[espindola]

One case to consider is conditions in .rc file. For example, using gtk on linux and win32 on windows.

[brson]

Closing this. There are a few remaining known issues but I'll open new bugs for them.