clarify under what conditions you can compile without GDAL

git-svn-id: http://svn.osgeo.org/postgis/trunk@11329 b70326c6-7e19-0410-871a-916f4a2858ee
one more clarification

git-svn-id: http://svn.osgeo.org/postgis/trunk@11330 b70326c6-7e19-0410-871a-916f4a2858ee