fixup std::abs in map_location.hpp

The reason that OS X wasn't finding std::abs(int) is that we
weren't including <cstdlib>, which is where it resides, not
<cmath>. This commit adds cstdlib, and reverts the other fix
attempts.

This fixes up two previous commits:

b05773c4099e9a2c987adc0c656ec7ed96dec0ff

7ce9199e470025b69935812d8445fc280767fb42