修复lua5.4.6，构建iOS平台时出现报错（'system' is unavailable: not available on iOS） (#1133)

Author: ClaineLe

build/CMakeLists.txt

@@ -145,6 +145,7 @@ source_group_by_dir(${CMAKE_CURRENT_SOURCE_DIR} XLUA_CORE)
 
 if (APPLE)
     if (IOS)
+        ADD_DEFINITIONS(-DLUA_USE_IOS) # Despite claiming to be ISO C, iOS does not implement 'system'.(iOS11)
         set(CMAKE_OSX_ARCHITECTURES "$(ARCHS_STANDARD)")
         add_library(xlua STATIC
            ${LUA_CORE}