When we build neither any system emulation targets nor the tools there
is actually no need for pixman library.  In that case do not enforce
presence of that library on the system.
Reviewed-by: NAndreas F=E4rber <afaerber@suse.de>
Signed-off-by: NRobert Schiele <rschiele@gmail.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>