ENH: add a better message

Modules/FindQt.cmake

@@ -129,7 +129,9 @@ ELSE(NOT QT3_INSTALLED AND NOT QT4_INSTALLED)
   ENDIF(NOT QT_FOUND AND NOT DESIRED_QT_VERSION)
   IF(NOT QT_FOUND AND DESIRED_QT_VERSION)
     IF(QT_REQUIRED)
-      MESSAGE(SEND_ERROR "CMake was unable to find QT version: ${DESIRED_QT_VERSION}. Set advanced values QT_QMAKE_EXECUTABLE and QT${DESIRED_QT_VERSION}_QGLOBAL_FILE.")
+      IF(NOT QT_QT_LIBRARY)
+        SET(MSG "QT_QT_LIBRARY was not found.
+      MESSAGE(SEND_ERROR "CMake was unable to find QT version: ${DESIRED_QT_VERSION}. Set advanced values QT_QMAKE_EXECUTABLE and QT${DESIRED_QT_VERSION}_QGLOBAL_FILE, if those are set then QT_QT_LIBRARY or QT_LIBRARY_DIR.")
     ELSE(QT_REQUIRED)
       MESSAGE( "CMake was unable to find desired QT version: ${DESIRED_QT_VERSION}. Set advanced values QT_QMAKE_EXECUTABLE and QT${DESIRED_QT_VERSION}_QGLOBAL_FILE.")
     ENDIF(QT_REQUIRED)