Append and do not clobber CMAKE_CXX_FLAGS in the test.

Tests/IncludeDirectories/CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required (VERSION 2.6)
 project(IncludeDirectories)
 
-set(CMAKE_CXX_FLAGS "\"-I${CMAKE_SOURCE_DIR}/Flags\"")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} \"-I${CMAKE_SOURCE_DIR}/Flags\"")
 include_directories(${CMAKE_SOURCE_DIR}/IncDir)
 set_source_files_properties(main.cpp PROPERTIES COMPILE_FLAGS
   "\"-I${CMAKE_SOURCE_DIR}/SrcProp\"")