documents portal: Make sure O_PATH is defined
documents portal: Make sure O_PATH is defined

FreeBSD doesn't have it.

https://bugzilla.gnome.org/show_bug.cgi?id=768780

johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib