I couldn't find any usage of these extern variables. neither in the file they are defined nor anywhere else in the code
#this file is included only when system Lua library is not found
file(GLOB lua_src *.c *.h)
if(WIN32)
add_definitions(-DLUA_BUILD_AS_DLL)
add_library(lua SHARED ${lua_src})
set(LUA_LIBRARY lua.dll)
set_target_properties(lua PROPERTIES PREFIX "")
install(TARGETS lua RUNTIME DESTINATION ${target_library_install_dir})
else(WIN32)
add_definitions(-DLUA_USE_LINUX)
add_library(lua STATIC ${lua_src})
set(LUA_LIBRARY lua)
endif(WIN32)