Updating the latest OPAM, small thing to note

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

If you are using the very latest OPAM and run into a compilation error, then 
just run 'make distclean' before starting a new compilation.

Some of the support packages have changed to support fancy git-style man-pages 
if you run 'opam install --help', and the Makefile dependencies aren't quite 
enough to detect that.

-anil