Some improvements and changes for Haskell Cabal support.
* HASKELL_ENABLE_LIBRARY_PROFILING and HASKELL_ENABLE_HADDOCK_DOCUMENTATION
  are "User-settable variables", not "Package-settable variables".
* Change to HASKELL_ENABLE_HADDOCK_DOCUMENTATION=yes by default.
* Add HASKELL_ENABLE_SHARED_LIBRARY("yes" by default), to enable shlib support.
* Add support for dynamically conditional PLIST entries handling for
  HASKELL_ENABLE_SHARED_LIBRARY and HASKELL_ENABLE_LIBRARY_PROFILING.

discussed with pho@ and szptvlfn@.

(obache)