[Bug libstdc++/79017] Old PowerMac G5, MacPorts GCC 5.4, C++11 and "std::log2 has not been declared"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79017

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
What's the error?