Use system diff when available (my implementation is horrible)

git-svn-id: http://svn.osgeo.org/postgis/trunk@11500 b70326c6-7e19-0410-871a-916f4a2858ee
Not all args with a dash are options, only those starting with one

git-svn-id: http://svn.osgeo.org/postgis/trunk@11501 b70326c6-7e19-0410-871a-916f4a2858ee