Add external defined CXX flags to configuration

cmake/ProjectDefinitions.cmake

@@ -2,7 +2,7 @@ set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 if(UNIX)
-    set(CMAKE_CXX_FLAGS "-Wall -Werror")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Werror")
 elseif(WIN32)
     set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} "-Wall" "-Werror")
 endif()