ENH: use correct variable for qmake

Modules/FindQt4.cmake

@@ -132,7 +132,7 @@ FIND_PROGRAM(QT_QMAKE_EXECUTABLE NAMES qmake PATHS
   ${QT4_PATHS}
 )
 IF(QT_QMAKE_EXECUTABLE)
-  EXEC_PROGRAM(${QMAKE_PATH} ARGS "-query QT_VERSION"
+  EXEC_PROGRAM(${QT_QMAKE_EXECUTABLE} ARGS "-query QT_VERSION"
     OUTPUT_VARIABLE QTVERSION)
   IF(QTVERSION MATCHES "4.*")
     SET(QT4_QMAKE_FOUND TRUE)