configure: use libplacebo pkg-config file to find optional components

So we don't rely on possibly outdated headers from a previous installation.

The variables in libplacebo have been around since ¹ which is in version 3.104.0 and above. We require version 4.192.

Also don't try to use vulkan with libplacebo if the user disabled vulkan in the configure call.

Extracted from !6981

1. https://github.com/haasn/libplacebo/commit/98b92cd6b47dc6ed95cc69e9bb4a51b79b44573b ↩