C's pow function refuses to work with variable exponent

Let's say I have the following code snippet:

int i; double value;
for(i = 0; i < CONSTANT; i++) {
  value = (double)pow(2, i);
}

Trying to compile this code yields an "undefined reference to `pow'" error.

Including or excluding math.h makes no difference, since it ends up being included anyway.

Raising 2.0 to a hardcoded power works okay, but everything fails if I substitute the exponent by an expression that contains i.

What am I doing wrong? Thanks.