Flag: sysroot

[steveklabnik]

This issue is part of https://github.com/steveklabnik/rustdoc/issues/125

--sysroot: override the system root

Using this flag looks like this:

$ rustdoc src/lib.rs --sysroot /path/to/sysroot

Similar to rustc --sysroot, this lets you change the sysroot rustdoc uses when compiling your code.

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Should we keep this flag or not?

[steveklabnik]

This flag seems important.

[euclio]

I wonder if this could be handled by RUSTFLAGS?