Qt header usage

Move the nine #undef's common to all qt_*.cpp sources into qt_pre.h.

Make "hack.h" usage consistent; always enclose withing 'extern "C {'
and '}' even though only some of the sources care.

win/Qt/qt_msg.cpp

@@ -7,15 +7,6 @@
 extern "C" {
 #include "hack.h"
 }
-#undef Invisible
-#undef Warning
-#undef index
-#undef msleep
-#undef rindex
-#undef wizard
-#undef yn
-#undef min
-#undef max
 
 #include "qt_pre.h"
 #include <QtGui/QtGui>