Add libsoup and osm-gps-map to Mac build instructions.

Also force py27-pygtk to use the quartz variant, since MacPorts
doesn't have variant dependency resolutions.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>