Ensure MANIFEST.in is complete

Also make sure it doesn't include stuff like coverage.xml, heh.
Fix build error with MSVC++ on Python 3

You could reproduce that on Linux with

  find -name '*.so' -delete
  CFLAGS="-pedantic -ansi" python3 setup.py

you'd get

  src/zope/hookable/_zope_hookable.c:234:3: warning: ISO C90 forbids mixed declarations and code [-pedantic]