[Checkins] [zopefoundation/zope.security] 2a5fba: Use features and an optional_build_ext

[GitHub]

  Branch: refs/heads/issue33b
  Home:   https://github.com/zopefoundation/zope.security
  Commit: 2a5fba13a04cea6119ba92fe38e4483f4cb5c7f6
      https://github.com/zopefoundation/zope.security/commit/2a5fba13a04cea6119ba92fe38e4483f4cb5c7f6
  Author: Jason Madden <jamadden at gmail.com>
  Date:   2017-09-11 (Mon, 11 Sep 2017)

  Changed paths:
    M CHANGES.rst
    M setup.py

  Log Message:
  -----------
  Use features and an optional_build_ext

Just like zope.interface (copied from there).

Fixes #33.

This should eliminate any chance of polluting (local) wheel caches if
PURE_PYTHON happens to be defined.