Build system fix distclean error for pixman

  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>

Makefile

@@ -286,7 +286,7 @@ distclean: clean
 	for d in $(TARGET_DIRS) $(QEMULIBS); do \
 	rm -rf $$d || exit 1 ; \
         done
-	test -f pixman/config.log && make -C pixman distclean
+	if test -f pixman/config.log; then make -C pixman distclean; fi
 
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