#2539, Check for json-c/json.h presence/usability before json/json.h


git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12347 b70326c6-7e19-0410-871a-916f4a2858ee
Update with latest changes


git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12348 b70326c6-7e19-0410-871a-916f4a2858ee