Apparently there is an abs function defined in <cstdlib>, which works
only on integers. We want the std::abs overloads defined in <cmath>,
which are used by TNL::abs.

clang rocks!

http://stackoverflow.com/questions/29136607/c-what-is-wrong-with-abs#29136774

The methods did not work with zero array size, so extra checks were
added.

Curiously clang complained about it, but nvcc didn't.