fix issue #56 - 64-bit CPython 2.5 does not include ctypes on Windows

The issue was originally introduced in commit
8cc97352831ba804cd55eec408b088553e0245d4 which resolved issue #47 (pyreadline
incompatibility) but forgot to fall back to the original default behaviour in
case the fix was not supported by the surrounding Python environment.

This should not cause any new pyreadline incompatibility issues because:
  1. fallback code does not use any Windows structure definitions that might
     conflict with those defined by pyreadline.
  2. pyreadline does not support Python 2.5.