CMake: don't use CMAKE_CXX_FLAGS for gcc warning levels, it only works for CMAKE_BUILD_TYPE=None

CMakeLists.txt

@@ -387,12 +387,12 @@ endif()
 
 # Compiler settings
 if (CMAKE_COMPILER_IS_GNUCC)
-    SET(CMAKE_CXX_FLAGS "-Wall -Wextra -Wno-unused-parameter -Wno-reorder -std=c++98 -pedantic -Wno-long-long ${CMAKE_CXX_FLAGS}")
+    set_property(GLOBAL APPEND_STRING PROPERTY COMPILE_FLAGS "-Wall -Wextra -Wno-unused-parameter -Wno-reorder -std=c++98 -pedantic -Wno-long-long")
 
     execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion
                 OUTPUT_VARIABLE GCC_VERSION)
     if ("${GCC_VERSION}" VERSION_GREATER 4.6 OR "${GCC_VERSION}" VERSION_EQUAL 4.6)
-        SET(CMAKE_CXX_FLAGS "-Wno-unused-but-set-parameter ${CMAKE_CXX_FLAGS}")
+        set_property(GLOBAL APPEND_STRING PROPERTY COMPILE_FLAGS "-Wno-unused-but-set-parameter")
     endif("${GCC_VERSION}" VERSION_GREATER 4.6 OR "${GCC_VERSION}" VERSION_EQUAL 4.6)
 elseif (MSVC)
     # Enable link-time code generation globally for all linking