When pixman is not installed (or too old), but virglrenderer is available
and "configure" has been run with "--disable-system", the build currently
aborts when trying to compile vhost-user-gpu (since it requires pixman).
Let's skip the build of vhost-user-gpu when pixman is not installed or
too old. Instead of adding CONFIG_PIXMAN, it is simpler to move the
detection to pixman.
Based on a patch by Thomas Huth. <thuth@redhat.com>
Fixes: 9b52b17ba5 ("configure: Allow to build tools without pixman")
Reported-by: Rafael Kitover <rkitover@gmail.com>
Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
b7612f45da