std::log2 not in c++11 on 10.6 BuiltBot?

[Michael Dickens]

One of my ports won't build on the 10.6 buildbot, because it claims the
following (snipped for brevity):
{{{
/opt/local/bin/g++-mp-4.9 [snip] -std=c++11 -o FILE.cc.o -c FILE.cc
FILE.cc: In constructor "FILE(std::vector<std::vector<float> >)":
FILE.cc:46:54: error: "log2" is not a member of "std"
               (unsigned)(((float)chip_seq[0].size())/std::log2((float)chip_seq.size()))),
}}}

According to every source I've found, std::log2 should exist in c++11 --
not before this, but as of c++11. The compiler goes on to suggest using
just "double log2(double)"; whatever.

Anybody know what's going on here? Thanks! - MLD