Deal with Cabal's deficiencies in linking dynamically.

With this change PKGBUILDs of libs create symbolic links to the DSO at
/usr/lib/ghc-7.6.3/shared.  They also executables that are part of library
packages with '-dynlib=deploy' to avoid the huge rpath put in place by Cabal's
default.

This change has to be accompanied by a change in the GHC package that

  1. puts links to all basic lib DSOs in /usr/lib/ghc-7.6.3/shared, and
  2. installs an ld.so configuration file that instruct it to look for DSOs in
  that location too.

Signed-off-by: Magnus Therning <magnus@therning.org>