The _CSTD macro evaluates to ::std if the EWL C library is compiled in the std namespace, and to nothing if the EWL C library is compiled in the global namespace.
_STD and _CSTD are meant to prefix C++ and C objects in such a way that you don't have to care whether or not the object is in std or not. For example:
_STD::cout, or _CSTD::size_t.