[cmake,release] Removes bogus link directory.

Somehow we ended up putting CMAKE_PROJECT_SOURCE_DIR/lib into the
linker path. There should not be any library there and it should
not be needed for linking anyway. Additionally it causes warnings
in dune-grid-howto. Therefore this patch simply removes it.

cmake/modules/DuneMacros.cmake

@@ -592,7 +592,6 @@ macro(dune_project)
   # set include path and link path for the current project.
   include_directories("${CMAKE_BINARY_DIR}")
   include_directories("${CMAKE_SOURCE_DIR}")
-  link_directories("${CMAKE_SOURCE_DIR}/lib")
   include_directories("${CMAKE_CURRENT_BINARY_DIR}")
   include_directories("${CMAKE_CURRENT_SOURCE_DIR}")
   add_definitions(-DHAVE_CONFIG_H)