few bugs in vmath.h

[epiclevanguy]

in function
template <typename T, int len>
static inline T angle(const vecN<T,len>& a, const vecN<T,len>& b)
{
return arccos(dot(a, b));
}
my g++ 5.4.0 tell me what he don't know what is a arccos.
also it seems to me that arkosinus should be taken from the scalar product divided by the product of the lengths of the vectors.
Did i miss something?