'new_compiler()' factory function.
Added 'runtime_library_dirs' list (for -R linker option) and methods
  to manipulate it.
Deleted some obsolete comments.
Added all the filename manglign methods: 'object_filenames()',
  'shared_object_filename()', 'library_filename()',
  'shared_library_filename()'.
Added 'spawn()' method (front end to the "real" spawn).