ENH: Add convenience for identifying Cocoa based Qt.

Modules/FindQt4.cmake

@@ -591,6 +591,9 @@ IF (QT4_QMAKE_FOUND)
   CHECK_SYMBOL_EXISTS(Q_WS_WIN "QtCore/qglobal.h" Q_WS_WIN)
   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)
+    CHECK_SYMBOL_EXISTS(QT_MAC_USE_COCOA "QtCore/qconfig.h" QT_MAC_USE_COCOA)
+  ENDIF(Q_WS_MAC)
 
   IF (QT_QTCOPY_REQUIRED)
      CHECK_SYMBOL_EXISTS(QT_IS_QTCOPY "QtCore/qglobal.h" QT_KDE_QT_COPY)