Test floating point classify/compare support in <cmath>

Implement compile/run test of the C++11 floating point classification
and comparison functions/macro constants in the <cmath> header. If
test passes, set CMake cache variable HAS_CXX11_CMATH_FPCLASSIFY to
true (false otherwise). Document this variable in the CheckCXX11Features
module, and link to corresponding FNAL cpp0x definition in README.

NB: This check is implemented using the partition of cmath functionality
used in FNAL cpp0x. It may not be optimal for general C++11 checks.
Add algorithm/cmath definitions to example header