Rustdoc doesn't check for pandoc

[MaikKlein]

I am on Linux.
If I try to use rustdoc without an installed version of pandoc, it will get stuck in an infinite loop.

rustc 0.6 (35655a0 2013-05-29 13:29:06 -0700)

[thestinger]

This is still an issue with the latest master.

[thestinger]

Closing in favour of #9081.