Check if cmath supplies C99 functions in std::

Implement compile/run test for presence and basic functionality of
C99 function presence in std:: namespace when including <cmath>
header. If test passes, set CMake variable HAS_CXX11_CMATH_C99 to
true (false otherwise). Document variable in CheckCXX11Features module,
and link to corresponding FNAL cpp0x variable in README.

NB: test does not truly exercise functions for numerical correctness.