Check if <cstddef> provides std::nullptr_t

Implement simple compile/run and compile fail tests for presence and
functionality of std::nullptr_t in <cstddef>. If tests pass, set
CMake variable HAS_CXX11_CSTDDEF_NULLPTR_T to true (false otherwise).
Document this variable in the CheckCXX11Features module, and link
this to the corresponding definition used by the FNAL cpp0x code.