buildlink dependencies for libgnomecanvasmm and gtkmm

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

graphics/libgnomecanvasmm/buildlink3.mk includes
./../x11/gtkmm/buildlink3.mk

which has:

# increase API_DEPENDS as a hack because now depends on pangomm
BUILDLINK_API_DEPENDS.gtkmm+=   gtkmm>=2.18.2

Ignore that old comment -- it was from me and is no longer applicable 
since it has been bumped since then.

But I have libgnomecanvasmm-2.26.0nb1 installed which requires 
gtkmm>=2.14.1. My installed gtkmm-2.16.0 is good enough.

So the buildlinking fails when I build something that includes 
libgnomecanvasmm:

===> Creating toolchain wrappers for gtkpod-0.99.14nb2
ERROR: gtkmm>=2.18.2 is not installed; can't buildlink files.
*** Error code 1

This similar to
http://mail-index.netbsd.org/tech-pkg/2009/12/19/msg004553.html