Properly check for X11 backend to avoid compilation break about intel-vaapi-driver

[yakuizhao]

I don't think that it is a bug. Instead it is related with the system configuration.
In fact if the libva is built without x11 supported, it will disable libva-x11. Then the intel-vaapi-driver can be built with X11 disabled automatically.
But the building system has the libva-x11.pc. In such case it causes that USE_X11 is still configured as 1.
Then the error is triggered.

But it is not the driver bug.

[chivakker]

The point here is which library needs x11 to compile. If it is needed to compile then configure should be doing the check. libva was doing it because the test folder required it, so in that scenario all libraries were covered. Once libva-utils is introduced, the expectation is that it should be compiled in this order libva, intel-vaapi-driver and the libva-utils. libva doesn't need to check for x11 stuff anymore, so the driver and libva-utils should do it as they actually need it for compilation.

[yakuizhao]

The driver has the dependency on libva-x11. If the libva is built without libva-x11, the x11 is automatically disabled when trying to build the driver.(Although x11 is enabled for libva by default, it will also be disabled if the X11 library doesn't meet with the requirement.)

So if the libva has no libva-x11 library, the mentioned issue in driver is related with the system configuration.

BTW: libva-x11 is not only required by libva tests, but also is required by the windows system when running the libva.

[xhaihao]

@yakuizhao Even though user installs libva & libva-x11, it is possible not to install the develop package for X11, however we need the develop package fox X11 (e.g. libx11-dev on Ubuntu) to build the driver.

[yakuizhao]

@chivakker @xhaihao Thanks for the more detailed explanation. Sorry that I am wrong. If the libva is installed by the distribution package with libva-x11, the driver building error will be triggered if X11 library doesn't exist.