Configuration-time search for xrandr headers

[MathiasMagnus]

**What enhancement are you suggesting for the Vulkan Loader? Please describe in detail. **

During configuation, when BUILD_WSI_XLIB_SUPPORT is enabled, then pkg_check_modules(X11 REQUIRED QUIET IMPORTED_TARGET x11) searches for headers provided by libx11-dev, but it also set the VK_USE_PLATFORM_XLIB_XRANDR_EXT pre-processor definition. That in turn requires headers from a different package, libxrandr-dev. Missing this last library should not cause a build-time error, but a configuration-time one.

Is this specific to a single platform?
Linux

Additional context

[charles-lunarg]

Yes, this is a deficiency of the current include setup. I can pretty easily rectify the issue by putting the xrandr stuff in its own if-else block with its own cmake variable to control it. Since it has been required for long time, I will make the default to be "ON" but the new variable will make it easy to disable it.