GNotification: add priority

https://bugzilla.gnome.org/show_bug.cgi?id=731623
gcredentials: add NetBSD support

https://bugzilla.gnome.org/show_bug.cgi?id=728256
Change gio/data-to-c.pl to /usr/bin/env perl.

On FreeBSD perl gets installed in /usr/local, And a compat symlink in
/usr/bin. However this symlink might go away in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=724986