Add -DQT_NO_DEBUG if no build type is specified so Qt plugins will work.

Modules/UseQt4.cmake

@@ -21,6 +21,9 @@ SET_PROPERTY(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS_DEBUG QT_DEBUG)
 SET_PROPERTY(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS_RELEASE QT_NO_DEBUG)
 SET_PROPERTY(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS_RELWITHDEBINFO QT_NO_DEBUG)
 SET_PROPERTY(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS_MINSIZEREL QT_NO_DEBUG)
+IF(NOT CMAKE_CONFIGURATION_TYPES AND NOT CMAKE_BUILD_TYPE)
+  SET_PROPERTY(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS QT_NO_DEBUG)
+ENDIF()
 
 INCLUDE_DIRECTORIES(${QT_INCLUDE_DIR})