[BUG] FittingCurve() could just use log2() now?

[enh-google]

the NDK removed gnustl in 2017, so this can probably just be replaced with std::log2() now?

    case oLogN:
      /* Note: can't use log2 because Android's GNU STL lacks it */
      return [](IterationCount n) {
        return kLog2E * std::log(static_cast<double>(n));
      };
    case oNLogN:
      /* Note: can't use log2 because Android's GNU STL lacks it */
      return [](IterationCount n) {
        return kLog2E * static_cast<double>(n) *
               std::log(static_cast<double>(n));
      };

(as the Android libc maintainer, i can tell you that the underlying libm log2() has been present since API level 18, which is older than the lowest API level still supported by the NDK [which is 21].)