Only enable the automoc test after checking that Qt4 works

On some systems which contribute nightly builds there were strange
errors which seemed to hint that the installed Qt4 is not usable/
not usable with this compiler. So first check whether it works,
and only if this was successful, enable the test.

Alex

Tests/CMakeLists.txt

@@ -845,18 +845,37 @@ ${CMake_BINARY_DIR}/bin/cmake -DVERSION=master -P ${CMake_SOURCE_DIR}/Utilities/
   ENDIF(NOT QT4_FOUND)
 
   IF(QT4_FOUND)
-    ADD_TEST(QtAutomoc ${CMAKE_CTEST_COMMAND}
-      --build-and-test
-      "${CMake_SOURCE_DIR}/Tests/QtAutomoc"
-      "${CMake_BINARY_DIR}/Tests/QtAutomoc"
-      --build-generator ${CMAKE_TEST_GENERATOR}
-      --build-project QtAutomoc
-      --build-makeprogram ${CMAKE_TEST_MAKEPROGRAM}
-      --build-exe-dir "${CMake_BINARY_DIR}/Tests/QtAutomoc"
-      --force-new-ctest-process
-      --test-command ${CMAKE_CTEST_COMMAND} -V
-      )
-    LIST(APPEND TEST_BUILD_DIRS "${CMake_BINARY_DIR}/Tests/QtAutomoc")
+    # test whether the Qt4 which has been found works, on some machines
+    # which run nightly builds there were errors like "wrong file format"
+    # for libQtCore.so. So first check it works, and only if it does add
+    # the automoc test.
+    INCLUDE(CheckCXXSourceCompiles)
+    SET(_save_CMAKE_REQUIRED_INCLUDES "${CMAKE_REQUIRED_INCLUDES}")
+    SET(_save_CMAKE_REQUIRED_LIBRARIES "${CMAKE_REQUIRED_LIBRARIES}")
+
+    SET(CMAKE_REQUIRED_INCLUDES ${QT_INCLUDES})
+    SET(CMAKE_REQUIRED_LIBRARIES ${QT_QTCORE_LIBRARIES})
+
+    CHECK_CXX_SOURCE_COMPILES("#include <QCoreApplication>\n int main() {return (qApp == 0 ? 0 : 1); }\n"
+                              QT4_WORKS_FOR_AUTOMOC_TEST)
+
+    SET(CMAKE_REQUIRED_INCLUDES "${_save_CMAKE_REQUIRED_INCLUDES}")
+    SET(CMAKE_REQUIRED_LIBRARIES "${_save_CMAKE_REQUIRED_LIBRARIES}")
+
+    IF(QT4_WORKS_FOR_AUTOMOC_TEST)
+      ADD_TEST(QtAutomoc ${CMAKE_CTEST_COMMAND}
+        --build-and-test
+        "${CMake_SOURCE_DIR}/Tests/QtAutomoc"
+        "${CMake_BINARY_DIR}/Tests/QtAutomoc"
+        --build-generator ${CMAKE_TEST_GENERATOR}
+        --build-project QtAutomoc
+        --build-makeprogram ${CMAKE_TEST_MAKEPROGRAM}
+        --build-exe-dir "${CMake_BINARY_DIR}/Tests/QtAutomoc"
+        --force-new-ctest-process
+        --test-command ${CMAKE_CTEST_COMMAND} -V
+        )
+      LIST(APPEND TEST_BUILD_DIRS "${CMake_BINARY_DIR}/Tests/QtAutomoc")
+    ENDIF()
   ENDIF()
 
   ADD_TEST(ExternalProject ${CMAKE_CTEST_COMMAND}