Change deprecation warning to raise a WARNING after 2 version deprecation time

I kept the DEBUG for the first deprecating version to keep changes
small (no need to change testsuite again) but I really think
it should be at least a NOTICE in that case, or nobody would notice.

See #2440

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@11987 b70326c6-7e19-0410-871a-916f4a2858ee