Make boost_system optional as boost before 1.35.0 doesn't have it and mapnik seems to compile fine without it.
Make boost_system optional as boost before 1.35.0 doesn't have it and
mapnik seems to compile fine without it.


git-svn-id: 06a2cde72a096d93ff72b88e3e6711b8f00772a4@721 d397654b-2ff0-0310-9e29-ba003691a0f9

semprebon/mapnik,semprebon/mapnik,semprebon/mapnik,semprebon/mapnik,semprebon/mapnik