Currently Makefile test if pixman have configure log, but the script directly
return error if that file do not exist. This patch fix it.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>