Support ICC and older versions of GCC

Previously some versions of gcc (I tested 4.4.6 and 4.4.7) would give the following error:

```
lib/bench.hh: In function ‘T prime_lower_bound(T)’:
lib/bench.hh:163: error: there are no arguments to ‘sqrt’ that depend on a template parameter, so a declaration of ‘sqrt’ must be available
lib/bench.hh:163: note: (if you use ‘-fpermissive’, G++ will accept your code, but allowing the use of an undeclared name is deprecated)
```

Including math.h removes this error and also allows ICC to build the code.