[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 #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
OK, thanks. I'm not going to try to add declarations like the libc++ patch
does, as it looks like that would only be correct for Snow Leopard and not
earlier.

I'll just add more fine-grained configure checks and if they're not declared in
<math.h> then they won't be available, period. There's only so much we can do
to work around buggy C libraries on the target.