patch applied (cabal): Add mandir support to InstallDirs

[Duncan Coutts]

Fri Dec  7 06:18:39 PST 2007  Duncan Coutts <duncan at haskell.org>
  * Add mandir support to InstallDirs
  It's not used by Cabal at all but some packages want to install man pages
  and this should make it easier for them to figure out where to install them.
  In particular c2hs and lhs2tex have man pages.

    M ./Distribution/Simple/InstallDirs.hs +7