patch applied (cabal): PackageDescription.hs: add check for existence of license file

[Duncan Coutts]

Sat Jan 26 19:10:33 PST 2008  Brent Yorgey <byorgey at gmail.com>
  * PackageDescription.hs: add check for existence of license file
  Added a check for the existence of the license file (if one is
  specified in a license-file: field) during description file
  sanity checking.  If a license is specified but does not exist,
  a warning is generated.  This patch resolves issue #180.

    M ./Distribution/PackageDescription.hs -4 +18