ENH: add some checks on the qmake install

Modules/FindQt4.cmake

@@ -145,7 +145,12 @@ IF(QT4_QMAKE_FOUND)
     EXEC_PROGRAM( ${QT_QMAKE_EXECUTABLE}
       ARGS "-query QT_INSTALL_LIBS"
       OUTPUT_VARIABLE QT_LIBRARY_DIR_TMP )
-    SET(QT_LIBRARY_DIR ${QT_LIBRARY_DIR_TMP} CACHE PATH "Qt library dir")
+    IF(EXISTS "${QT_LIBRARY_DIR_TMP}")
+      SET(QT_LIBRARY_DIR ${QT_LIBRARY_DIR_TMP} CACHE PATH "Qt library dir")
+    ELSE(EXISTS "${QT_LIBRARY_DIR_TMP}")
+      MESSAGE("Warning: QT_QMAKE_EXECUTABLE reported QT_INSTALL_LIBS as ${QT_LIBRARY_DIR_TMP}")
+      MESSAGE("Warning: ${QT_LIBRARY_DIR_TMP} does not exist, Qt must not be installed correctly.")
+    ENDIF(EXISTS "${QT_LIBRARY_DIR_TMP}")
   ENDIF(NOT QT_LIBRARY_DIR)
   
   IF (APPLE)
@@ -715,5 +720,8 @@ IF(QT4_QMAKE_FOUND)
   SET( QT_QT_LIBRARY "")
   SET( QT_QTMAIN_LIBRARY "")
 ELSE(QT4_QMAKE_FOUND)
-  MESSAGE("QT_QMAKE_EXECUTABLE set to qmake version: QTVERSION = ${QTVERSION}\nQT_QMAKE_EXECUTABLE = ${QT_QMAKE_EXECUTABLE}, please set to path to qmake from qt4.")
+  IF(QT_QMAKE_EXECUTABLE)
+    MESSAGE("QT_QMAKE_EXECUTABLE set to qmake version: QTVERSION = ${QTVERSION}\nQT_QMAKE_EXECUTABLE = ${QT_QMAKE_EXECUTABLE}, please set to path to qmake from qt4.")
+  ENDIF(QT_QMAKE_EXECUTABLE)
+
 ENDIF(QT4_QMAKE_FOUND)