Build failure as isnan not declared with Ubuntu 16.04 + GCC 5.4.1
When build CMB against Ubuntu 16.04 and GCC 5.4.1, we would encounter an error as "error: ‘isnan’ was not declared in this scope". One possible solution is to use ifdef macro, however it would lead to error: ‘isnan’ is already declared in this scope error when building againt Unbuntu 14.04 (gcc 4.8). I have a potential patch to this bug, but it needs further testing on Ubuntu 14.04.