Maybe this example is not very correct. Let's say another example, you have determine the driver path via pkg-config as /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so on Ubuntu. While the driver could be changed in several ways:
1. Developer can build it self libva with different va driver path, for eg ~/va/dri/i965_drv_video_mine.so (developer can change that on libva configure time)
2. Ubuntu upgrade might change the driver, either parent_path(/usr/lib/x86_64-linux-gnu/dri/) or dirver name(i965_drv_video.so).

Either way won't work.

Anyway, I'd agree that pkg-config works in most scenarios than hard code solution. If you two agrees, I'll make patch works in that way.