doc: Add config check for doxygen 1.6.0+.

Add a config time check for a new enough (1.6.0+) version of doxygen.

v2. require 1.6.0+ instead of 1.8.0+

Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>