build: require Jansson if QEMU driver is enabled

If the QEMU driver was requested, require Jansson, since we need to use
the JSON monitor to probe capabilities for all QEMU version supported
by libvirt.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

m4/virt-driver-qemu.m4

@@ -27,6 +27,9 @@ AC_DEFUN([LIBVIRT_DRIVER_ARG_QEMU], [
 
 AC_DEFUN([LIBVIRT_DRIVER_CHECK_QEMU], [
   AC_REQUIRE([LIBVIRT_CHECK_JANSSON])
+  if test "$with_qemu:$with_jansson" = "yes:no"; then
+    AC_MSG_ERROR([Jansson >= 2.5 is required to build QEMU driver])
+  fi
   if test "$with_qemu" = "check"; then
     with_qemu=$with_jansson
   fi