- Removed dependency on cmake version 2.8.9
  Object libraries can now be created with dune_add_library(<target>
  OBJECT <sources>). It will create a GLOBAL property
  _DUNE_TARGET_OBJECTS:<target>_ that records the full path to the
  source files. Theses can later be referenced by providing
  _DUNE_TARGET_OBJECTS:<target>_ as one of the sources to dune_add_library

[[Imported from SVN: r6987]]