Windows Speech API (SAPI) is now only supported with MSVC compiler.

qgroundcontrol.pri

@@ -133,7 +133,7 @@ win32 {
         -lmingw32 -lSDLmain -lSDL -mwindows
     
     INCLUDEPATH += $$BASEDIR/lib/sdl/include \
-                   C:\Program Files\Microsoft SDKs\Windows\v7.0\Include
+                   "C:\Program Files\Microsoft SDKs\Windows\v7.0\Include"
 
     debug {
         DESTDIR = $$BASEDIR/bin

src/GAudioOutput.cc

@@ -40,8 +40,8 @@ This file is part of the PIXHAWK project
 #include <ApplicationServices/ApplicationServices.h>
 #endif
 
-#ifdef _WIN32
-//#include <windows.h>
+// Speech synthesis is only supported with MSVC compiler
+#if _MSC_VER
 #include <sapi.h>
 using System;
 using System.Speech.Synthesis;
@@ -118,7 +118,8 @@ bool GAudioOutput::say(QString text, int severity)
     if (!emergency)
     {
 
-#ifdef _WIN32
+        // Speech synthesis is only supported with MSVC compiler
+#if _MSC_VER
         SpeechSynthesizer synth = new SpeechSynthesizer();
         synth.SelectVoice("Microsoft Anna");
         synth.SpeakText("Hello, world!");