Add support for disabling offline raster storage at compile time

Adds --disable-offline-raster configure switch

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12467 b70326c6-7e19-0410-871a-916f4a2858ee
Advertise --disable-offline-raster in NEWS

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12469 b70326c6-7e19-0410-871a-916f4a2858ee