Find -lX11 using PKG_CHECK_MODULES

This is better than simply hardcoding the path.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Do not include Xrender.h

This include was left over from when the terminal was using Xrender rather than
OpenGL.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>