GUACAMOLE-25: Fix configure test for FreeRDP variations.

configure.ac

@@ -779,7 +779,11 @@ then
                                             wStream* stream);
                                         IWTSVirtualChannelCallback cb = {
                                             .OnDataReceived = __data_received
-                                        };]])],
+                                        };
+                                        int main() {
+                                            return
+                                                cb.OnDataReceived(NULL, NULL);
+                                        }]])],
                       [AC_MSG_RESULT([yes])],
                       [AC_MSG_RESULT([no])
                        AC_DEFINE([LEGACY_IWTSVIRTUALCHANNELCALLBACK],,

src/protocols/rdp/guac_ai/ai_service.c

@@ -95,7 +95,7 @@ static void guac_rdp_ai_handle_data(guac_client* client,
 
 }
 
-#if LEGACY_IWTSVIRTUALCHANNELCALLBACK
+#ifdef LEGACY_IWTSVIRTUALCHANNELCALLBACK
 /**
  * Callback which is invoked when data is received along a connection to the
  * AUDIO_INPUT plugin. This callback is specific to FreeRDP 1.1 and older.