slirp: It needs to use QEMU_CFLAGS not CFLAGS

Signed-off-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -1861,7 +1861,7 @@ if test $profiler = "yes" ; then
 fi
 if test "$slirp" = "yes" ; then
   echo "CONFIG_SLIRP=y" >> $config_host_mak
-  CFLAGS="-I\$(SRC_PATH)/slirp $CFLAGS"
+  QEMU_CFLAGS="-I\$(SRC_PATH)/slirp $QEMU_CFLAGS"
 fi
 if test "$vde" = "yes" ; then
   echo "CONFIG_VDE=y" >> $config_host_mak