resolve ambiguities in referring to std::abs, in map_location.hpp

Ancestral reported compilation errors with clang on OS X,
basically similar to what is reported here:

https://github.com/Project-OSRM/osrm-backend/issues/1000

I fix it by replacing calls to std::abs, with calls to _abs, a
function pointer I define to be & _abs, so that I can control
exactly which overload is selected and thus resolve the ambiguity.