Rustdoc could allow setting compilation flags for compiling, or just a no_link flag

[huonw]

E.g. exhibiting #[no_std]; code normally requires passing extra flags to rustc (because we don't link to libc), so it would be good to be able to just parse, type-check and lint without linking, so that code snippets don't go completely out of date.

Maybe something like

```no_link
#[no_std];
#[start]
fn main(_argc: int, _argv: **u8) -> int { 0 }
```

[alexcrichton]

Perhaps no_run could imply --no-trans?

[flaper87]

I think I prefer @huonw proposal. That will help keeping the code fence explicit.