Added 'link_static_lib()' method, and 'archiver' and 'archiver_options'
  class attributes to support it.
Added 'link_executable()' method, and 'ld_exec' instance attribute
  to support it.
'newer_group()' is now able to handle missing files, so we don't have
  to kludge it by catching OSError when calling it.
'object_filenames()' and 'shared_object_filename()' now take 'keep_dir'
  flag parameters.
'library_filename()' and 'shared_library_filename()' now respect
  a directory component in the library name.
Various comment updates/deletions.