configure: check for pixman-1 version

commit a93a3af introduces use of PIXMAN_TYPE_RGBA, but it's only available
in pixman >= 0.21.8. If pixman doesn't meet the version requirement, qemu
will fail to build with following message:

qemu/ui/qemu-pixman.c: In function ‘qemu_pixelformat_from_pixman’:
qemu/ui/qemu-pixman.c:42: error: ‘PIXMAN_TYPE_RGBA’ undeclared (first use in this function)
qemu/ui/qemu-pixman.c:42: error: (Each undeclared identifier is reported only once
qemu/ui/qemu-pixman.c:42: error: for each function it appears in.)

This patch fixes the problem by checking the pixman version.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

configure

@@ -2737,7 +2737,7 @@ fi
 if test "$pixman" = ""; then
   if test "$want_tools" = "no" -a "$softmmu" = "no"; then
     pixman="none"
-  elif $pkg_config pixman-1 > /dev/null 2>&1; then
+  elif $pkg_config --atleast-version=0.21.8 pixman-1 > /dev/null 2>&1; then
     pixman="system"
   else
     pixman="internal"
@@ -2753,11 +2753,12 @@ if test "$pixman" = "none"; then
   pixman_cflags=
   pixman_libs=
 elif test "$pixman" = "system"; then
+  # pixman version has been checked above
   pixman_cflags=`$pkg_config --cflags pixman-1`
   pixman_libs=`$pkg_config --libs pixman-1`
 else
   if test ! -d ${source_path}/pixman/pixman; then
-    error_exit "pixman not present. Your options:" \
+    error_exit "pixman >= 0.21.8 not present. Your options:" \
         "  (1) Preferred: Install the pixman devel package (any recent" \
         "      distro should have packages as Xorg needs pixman too)." \
         "  (2) Fetch the pixman submodule, using:" \