Fix readline build error occurring when targeting MacOSX SDK 10.4

This commit integrates commit python/cpython@7baeed457 and fixes the
following error:

Python-2.7.3/Modules/readline.c:1026: error: 'rl_event_hook' undeclared (first use in this function)

Fixes #31
Merge branch '31-fix-readline-build-error-macosx104'

* 31-fix-readline-build-error-macosx104:
  Fix readline build error occurring when targeting MacOSX SDK 10.4