Add flag needed on OSX 10.7 to tell clang to use libc++ instead of
libstdc++.  Also export that flag for others to use.
Merge branch 'master' of github.com:osrf/ros_dds into use_libc++
Use libc++ instead of libstdc++.
Merge pull request #14 from osrf/use_libc++

Use libc++ (needed for older clangs)