- New lofar_package() macro replaces lofar_add_package(). The new macro
  supports recursive addition of LOFAR packages, using the dependencies passed
  as arguments. For example: 
    lofar_package(Blob 1.0 DEPENDS Common) 
  will add the package Common to the build (unless it was already added).
  This new features enables a "top-down" package specification order, which is
  required for building subsystems.

- New lofar_add_package() macro replaces lofar_add_subdirectory(). This macro
  uses a ${pkg}_SOURCE_DIR variable to locate the source directory of the
  package <pkg>. This information is needed, as we cannot simply include
  actual subdirectories anymore, because dependent package(s) might be located
  in a different directory tree.

- New file LofarPackageList.cmake, which sets the ${pkg}_SOURCE_DIR variables.
  This file is generated by a script gen_LofarPackageList_cmake.sh, which must
  be rerun when LOFAR packages are added, moved, or removed.