Driver installation should use libva.pc driverdir variable

[uartie]

LibVA provides a pkgconfig (libva.pc) file that defines a driverdir variable. This variable specifies the default path that libva will search for drivers. This variable can be obtained with pkg-config libva --variable driverdir. The cmake configuration should use this driverdir as the default installation location for the driver (but still allow it to be overridden by developer... e.g. -DLIBVA_DRIVERS_PATH=). Obviously, users can still override libva driver search path with environment var at runtime.

[oliver-sang]

thanks for reporting this issue and thanks for the PR. I ran into some problem while trying your PR which I added comments in PR. Could you help? Thanks