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

dbus/message.h

Index: dbus/message.h
diff --git a/dbus/message.h b/dbus/message.h
index 4d32cb9bae36edbae6915ef9d587dadfabd69c70..85d045c2812b417dd3475d57c4921d885c514f49 100644
--- a/dbus/message.h
+++ b/dbus/message.h
@@ -28,13 +28,15 @@ 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.
 //