--osx-version-min has default value 10.6, which breaks building with libc++. We could choose anything between 10.7 and 10.9 I believe. Would there be any reason to choose somehting less that 10.9 on mavericks for this formula?

libc++ is not on by default in the sconscript of mongodb, but it might be for Mavericks clang. So yeah, maybe better remove it and default to the system default?