autogen.sh: allow calling from out-of-tree
autogen.sh: allow calling from out-of-tree

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>

https://bugzilla.gnome.org/show_bug.cgi?id=692857

Distrotech/gst-plugins-ugly,Distrotech/gst-plugins-ugly,Distrotech/gst-plugins-ugly,Distrotech/gst-plugins-ugly,Distrotech/gst-plugins-ugly