- added --enable-localdune option

- added two types of library-check:
  a) normal search for lib in default-path (for a global installation
     some time in the future)
  b) a check for a signs of a compiled lib in a local
     Dune-installation if "localdune" is enabled. This is by no chance
     a strict check but what we want for local development

[[Imported from SVN: r251]]