Change dbus check for file capability to runtime (second try)

dbus/message.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DBUS_MESSAGE_H_
 #define DBUS_MESSAGE_H_
 
 #include <string>
 #include <vector>
 #include <dbus/dbus.h>
@@ skipping 10 matching lines @@
 }  // namespace protobuf
 }  // namespace google
 
 
 namespace dbus {
 
 class MessageWriter;
 class MessageReader;
 
 // DBUS_TYPE_UNIX_FD was added in D-Bus version 1.4
-#if defined(DBUS_TYPE_UNIX_FD)
-const bool kDBusTypeUnixFdIsSupported = true;
-#else
-const bool kDBusTypeUnixFdIsSupported = false;
+#if !defined(DBUS_TYPE_UNIX_FD)
 #define DBUS_TYPE_UNIX_FD      ((int) 'h')
 #endif
 
+// Returns true if Unix FD passing is supported in libdbus.
+// The check is done runtime rather than compile time as the libdbus
+// version used at runtime may be different from the one used at compile time.
+bool IsDBusTypeUnixFdSupported();
+
 // Message is the base class of D-Bus message types. Client code must use
 // sub classes such as MethodCall and Response instead.
 //
 // The class name Message is very generic, but there should be no problem
 // as the class is inside 'dbus' namespace. We chose to name this way, as
 // libdbus defines lots of types starting with DBus, such as
 // DBusMessage. We should avoid confusion and conflict with these types.
 class Message {
  public:
   // The message type used in D-Bus.  Redefined here so client code
@@ skipping 425 matching lines @@
 
   Message* message_;
   DBusMessageIter raw_message_iter_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageReader);
 };
 
 }  // namespace dbus
 
 #endif  // DBUS_MESSAGE_H_