* 'build_dir' -> 'build_lib', which by default takes its value
    straight from 'build_lib' in the 'build' command
  * added 'build_temp' and 'inplace' options
  * change 'build_extensions()' to put object files (compiler turds) in
    'build_temp' dir
  * complicated the name-of-extension-file shenanigans in
    'build_extensions()' to support "in-place" extension building, i.e.
    put the extension right into the source tree (handy for developers)
  * added 'get_ext_fullname()', renamed 'extension_filename()' to
    'get_ext_filename()', and tweaked the latter a bit -- all to support
    the new filename shenanigans