Fail silently if system Qt installation is broken

Fixes a regression in behaviour from 2.8.10 to 2.8.11 resulting in the
following error if the system Qt installation is broken:

CMake Error at Modules/FindQt4.cmake:1028 (set_property):
  set_property could not find TARGET Qt4::QtCore.  Perhaps it has not yet
  been created.
Call Stack (most recent call first):
  Tests/RunCMake/CMakeLists.txt:79 (find_package)

Upstream-Status: Pending

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

--- cmake-2.8.11/Modules/FindQt4.cmake	2013-07-04 10:20:42.092848274 +0100
+++ cmake-2.8.11/Modules/FindQt4.cmake	2013-07-04 10:52:15.693380555 +0100
@@ -1025,21 +1025,22 @@
     set(_configs)
   endmacro()
 
-
-  # Set QT_xyz_LIBRARY variable and add
-  # library include path to QT_INCLUDES
-  _QT4_ADJUST_LIB_VARS(QtCore)
-  set_property(TARGET Qt4::QtCore APPEND PROPERTY
-    INTERFACE_INCLUDE_DIRECTORIES
-      "${QT_MKSPECS_DIR}/default"
-      ${QT_INCLUDE_DIR}
-  )
-  set_property(TARGET Qt4::QtCore PROPERTY
-    INTERFACE_QT_MAJOR_VERSION 4
-  )
-  set_property(TARGET Qt4::QtCore APPEND PROPERTY
-    COMPATIBLE_INTERFACE_STRING QT_MAJOR_VERSION
-  )
+  if(QT_QTCORE_FOUND)
+    # Set QT_xyz_LIBRARY variable and add
+    # library include path to QT_INCLUDES
+    _QT4_ADJUST_LIB_VARS(QtCore)
+    set_property(TARGET Qt4::QtCore APPEND PROPERTY
+      INTERFACE_INCLUDE_DIRECTORIES
+        "${QT_MKSPECS_DIR}/default"
+        ${QT_INCLUDE_DIR}
+    )
+    set_property(TARGET Qt4::QtCore PROPERTY
+      INTERFACE_QT_MAJOR_VERSION 4
+    )
+    set_property(TARGET Qt4::QtCore APPEND PROPERTY
+      COMPATIBLE_INTERFACE_STRING QT_MAJOR_VERSION
+    )
+  endif()
 
   foreach(QT_MODULE ${QT_MODULES})
     _QT4_ADJUST_LIB_VARS(${QT_MODULE})