common: don't include <math.h> or <cmath> if already available
common: don't include <math.h> or <cmath> if already available

We don't want to include <cmath> because it will undefine most of the
macros in <math.h>, and we don't want to include <math.h> if we already
have <cmath> since it will define those macros and apparently the code
using SIMDe prefers <cmath>.

nemequ/simde,nemequ/simde,nemequ/simde