Comment 2 for bug 1831385

#include <cmath>

using std::fabsf;
using std::fabsl;

abs.cc:3:12: error: ‘std::fabsf’ has not been declared
 using std::fabsf;
            ^~~~~
abs.cc:4:12: error: ‘std::fabsl’ has not been declared
 using std::fabsl;
            ^~~~~

These are required by C++17 (and maybe implicitly by C++11, although it's not very clear)