ENH: Do a recheck of QT_MAC_USE_COCOA when qmake executable changes.

Modules/FindQt4.cmake

@@ -669,6 +669,9 @@ IF (QT4_QMAKE_FOUND)
   CHECK_SYMBOL_EXISTS(Q_WS_QWS "QtCore/qglobal.h" Q_WS_QWS)
   CHECK_SYMBOL_EXISTS(Q_WS_MAC "QtCore/qglobal.h" Q_WS_MAC)
   IF(Q_WS_MAC)
+    IF(QT_QMAKE_CHANGED)
+      UNSET(QT_MAC_USE_COCOA CACHE)
+    ENDIF(QT_QMAKE_CHANGED)
     CHECK_SYMBOL_EXISTS(QT_MAC_USE_COCOA "QtCore/qconfig.h" QT_MAC_USE_COCOA)
   ENDIF(Q_WS_MAC)