mapnik: fix build against Boost 1.55 and libc++

Avoids these issue for Mavericks users until Mapnik provides new upstream version:

 - https://github.com/mapnik/mapnik/issues/2082
 - https://github.com/mapnik/mapnik/issues/1973

Closes #24411.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>