If pixman is not installed, it is currently not possible to run:

 .../configure  --disable-system --enable-tools

Seems like there was a dependency from one of the required source
files to pixman in the past, but since commit 1ac0206b
("qemu-timer.c: Trim list of included headers"), this dependency
should be gone. Thus allow to compile the tools without pixman now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20200723141123.14765-1-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>