- 'export_symbol_file' (and corresponding 'def_file' in the old
    "build info" dict) are gone; warn if we see 'def_file' in the
    dict
  - the MSVC "pre-link hack" is gone -- all that stuff is now handled
    elsewhere (eg. by using 'export_symbols', etc.)
  - add 'get_export_symbols()' and 'get_libraries()' methods -- needed
    because on Windows, both of those things are a tad more complicated
    than fetching them from the Extension instance