Report libjson version from postgis_full_version(), closing #2159

git-svn-id: http://svn.osgeo.org/postgis/trunk@12652 b70326c6-7e19-0410-871a-916f4a2858ee
Document minimum required libjson-c version being 0.10

Latest debian stable (wheezy) packages 0.11 already.
See http://trac.osgeo.org/postgis/ticket/2539#comment:6

git-svn-id: http://svn.osgeo.org/postgis/trunk@12653 b70326c6-7e19-0410-871a-916f4a2858ee