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>