Don't use system include path

Don't use system include path

Sometimes systems have _old_ DB_DataObject classes lying around that
get included by default, so we just try to avoid anything that we don't
ship ourselves.

<MMN-o> BeS: I'll commit a patch that will make this issue go away
<BeS> MMN-o: that would be awesome!
<MMN-o> but it might upset bashrc who's working on a Debian package (where you're _supposed_ to include from /usr/php etc. :P)
<MMN-o> but I'll leave a comment along with it