build: fix build with older dbus headers

Compilation on a RHEL 5 host failed, due to the older dbus headers
present on that machine, and triggered by commit 2aa167ca:

util/virdbus.c: In function 'virDBusMessageIterDecode':
util/virdbus.c:952: error: 'DBusBasicValue' undeclared (first use in this function)

* m4/virt-dbus.m4 (LIBVIRT_CHECK_DBUS): Check for DBusBasicValue.
* src/util/virdbuspriv.h (DBusBasicValue): Provide fallback.
Signed-off-by: NEric Blake <eblake@redhat.com>

m4/virt-dbus.m4

 dnl The libdbus.so library
 dnl
-dnl Copyright (C) 2012-2013 Red Hat, Inc.
+dnl Copyright (C) 2012-2014 Red Hat, Inc.
 dnl
 dnl This library is free software; you can redistribute it and/or
 dnl modify it under the terms of the GNU Lesser General Public
@@ -26,6 +26,7 @@ AC_DEFUN([LIBVIRT_CHECK_DBUS],[
     CFLAGS="$CFLAGS $DBUS_CFLAGS"
     LIBS="$LIBS $DBUS_LIBS"
     AC_CHECK_FUNCS([dbus_watch_get_unix_fd])
+    AC_CHECK_TYPES([DBusBasicValue], [], [], [[#include <dbus/dbus.h>]])
     CFLAGS="$old_CFLAGS"
     LIBS="$old_LIBS"
   fi

src/util/virdbuspriv.h

 /*
  * virdbuspriv.h: internal APIs for testing DBus code
  *
- * Copyright (C) 2012-2013 Red Hat, Inc.
+ * Copyright (C) 2012-2014 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -24,6 +24,21 @@
 
 # include "virdbus.h"
 
+# if !HAVE_DBUSBASICVALUE
+/* Copied (and simplified) from dbus 1.6.12, for use with older dbus headers */
+typedef union
+{
+  dbus_int16_t  i16;   /**< as int16 */
+  dbus_uint16_t u16;   /**< as int16 */
+  dbus_int32_t  i32;   /**< as int32 */
+  dbus_uint32_t u32;   /**< as int32 */
+  dbus_bool_t   bool_val; /**< as boolean */
+  dbus_int64_t  i64;   /**< as int64 */
+  dbus_uint64_t u64;   /**< as int64 */
+  double dbl;          /**< as double */
+  unsigned char byt;   /**< as byte */
+} DBusBasicValue;
+# endif
 
 int virDBusMessageEncodeArgs(DBusMessage* msg,
                              const char *types,