Bug 735828 - configure fails for python < 2.7.

sys.version_info named components only available since 2.7, though GIMP
is supposed to support 2.5.0 and over. Used numerical tuple index
instead.
Simplify calls to g_output_stream_write_all()

It returns TRUE only if all bytes were written, so there is no need to
get and check the bytes_written return value.