build_docs failing

[awelkie]

running rake PLATFORM=lpc17xx build_docs fails with

error: couldn't find crate 'rlibc' with expected target triple x86_64-unknown-linux-gnu

obviously, rustc is looking for the x86_64 version of rlibc, which isn't being built.

[0xc0170]

Task build_docs_html does not include target definition for building lib.rs.

[farcaller]

It's not exactly trivial to fix, we need to keep various libs built for different targets.

[mcoffin]

Fixed by #285 and mcoffin/rlibc@8cb879f

The command cargo doc has replaced the old build_docs target from rake.