world.cc compile error: call of overloaded ‘abs(const double&)’ is ambiguous

[mjbogusz]

The problem originates in using C's stdlib.h (and its abs()) instead of 'proper' C++ cmath.
Occured with g++ 6.1.1, on Archlinux x86_64.

This patch solves the problem while modifying the code as less as possible: world_cc_abs.patch.txt

I've encountered another compile errors too but there's already a PR fixing them.

[rtv]

thanks for the fix. We use std::abs() explicitly now.