build: be more careful about glvnd

The opengl dependency was added to prefer glvnd when available. But doing that without specifying the method to pkg-config leads us to trying to use this on macos, which using the extraframework method will pick up OpenGL.Framework.

Let's avoid doing that, so macOS users can link to Mesa's libGL again. While we're at it, turn glvnd into a feature so we can opt in or out on any platform.

/cc @duncan.hopkins