In ticket SERVER-3829, the options -use-system-all, --use-system-pcre and --use-system-sm were added. In fact, mongodb 2.0.2's "scons --help" advertises that it also supports the option --use-system-snappy. And using "-use-system-all" reports:

using system version of: pcre
using system version of: sm
using system version of: snappy

Nevertheless, the bundled version of snappy is built and used instead of the system version I have installed with MacPorts.