Fix abs(size_t) Compile Error on Red Hat

### System
  Red Hat 4.4.7-3 + GCC 4.4.7
  (Scientific Linux on LBL's Lawrencium)

### Error message
  `DCHelper.hpp:151: error: call of overloaded 'abs(size_t)' is ambiguous`

`abs(size_t)` lives in <cstdlib> and is not in the `std::`
namespace [1]

[1] http://compgroups.net/comp.lang.c++.moderated/std-abs-int-ambiguous/195658