Compute $\int_{0}^{\pi} P^1_l(\cos(\theta))P^1_l(\cos(\theta-\alpha))\sin(\theta)d\theta$

Question

I wanna compute

$$\int_{0}^{\pi} P^1_l(\cos(\theta))P^1_l(\cos(\theta-\alpha))\sin(\theta)d\theta$$

Where $P^1_l(\cos(\theta))$ is the associated Legendre polynomial of order $1$ and degree $l$, and $\alpha$ is a constant.

I'm pretty much stuck, and can do nothing about it.