[Master] Use add_definitions

Because CMAKE_CXX_STANDARD introduced in CMake 3.1

apps/master/CMakeLists.txt

@@ -1,6 +1,8 @@
 project(masterserver)
 
-set(CMAKE_CXX_STANDARD 14)
+#set(CMAKE_CXX_STANDARD 14)
+add_definitions(-std=gnu++14)
+
 include_directories("./")
 
 set(SOURCE_FILES main.cpp MasterServer.cpp MasterServer.hpp RestServer.cpp RestServer.hpp)