Skip libxl driver on Xen 4.2

Specific to v0.9.11.  This is the opposite of commit dfa1e1dd,
and done for the purposes of minimal code changes to allow
compilation of libxl on Fedora 17 which still has experimental
libxl 4.1, while still being able to compile the branch (minus
libxl code) on Fedora 18 and later.
Signed-off-by: NEric Blake <eblake@redhat.com>

configure.ac

@@ -639,9 +639,16 @@ if test "$with_libxl" != "no" ; then
     CFLAGS="$CFLAGS $LIBXL_CFLAGS"
     LIBS="$LIBS $LIBXL_LIBS"
     AC_CHECK_LIB([xenlight], [libxl_domain_create_new], [
-        with_libxl=yes
+        dnl The v0.9.11-maint branch of libvirt is designed for Fedora 17, where
+        dnl libxl 4.1 was still experimental.  Make sure we don't try to compile
+        dnl against libxl 4.2 or greater (opposite what newer libvirt does).
+        AC_CHECK_LIB([xenlight], [libxl_ctx_alloc], [
+          if test "$with_libxl" = "yes"; then
+              fail=1
+          fi
+          with_libxl=no], [with_libxl=yes])
         LIBXL_LIBS="$LIBXL_LIBS -lxenlight -lxenstore -lxenctrl -lxenguest -luuid -lutil -lblktapctl"
-    ],[
+        ],[
         if test "$with_libxl" = "yes"; then
             fail=1
         fi