Taken from: https://github.com/awesomeWM/awesome/commit/d256d9055095f27a33696e0aeda4ee20ed4fb1a0
Author: Reiner Herrmann <reiner@reiner-h.de>

--- a/common/lualib.c
+++ b/common/lualib.c
@@ -20,6 +20,8 @@
 #include "common/lualib.h"
 #include "luaa.h"
 
+lua_CFunction lualib_dofunction_on_error;
+
 void luaA_checkfunction(lua_State *L, int idx)
 {
     if(!lua_isfunction(L, idx))
--- a/common/lualib.h
+++ b/common/lualib.h
@@ -28,7 +28,7 @@
 #include "common/util.h"
 
 /** Lua function to call on dofunction() error */
-lua_CFunction lualib_dofunction_on_error;
+extern lua_CFunction lualib_dofunction_on_error;
 
 void luaA_checkfunction(lua_State *, int);
 void luaA_checktable(lua_State *, int);
--- a/luaa.c
+++ b/luaa.c
@@ -91,6 +91,8 @@ extern const struct luaL_Reg awesome_mouse_meta[];
 extern const struct luaL_Reg awesome_root_methods[];
 extern const struct luaL_Reg awesome_root_meta[];
 
+signal_array_t global_signals;
+
 /** A call into the Lua code aborted with an error.
  *
  * This signal is used in the example configuration, @{05-awesomerc.md},
--- a/luaa.h
+++ b/luaa.h
@@ -317,7 +317,7 @@ const char *luaA_find_config(xdgHandle *, const char *, luaA_config_callback *);
 bool luaA_parserc(xdgHandle *, const char *);
 
 /** Global signals */
-signal_array_t global_signals;
+extern signal_array_t global_signals;
 
 int luaA_class_index_miss_property(lua_State *, lua_object_t *);
 int luaA_class_newindex_miss_property(lua_State *, lua_object_t *);
--- a/objects/button.c
+++ b/objects/button.c
@@ -35,6 +35,8 @@
 
 #include "button.h"
 
+lua_class_t button_class;
+
 /** Button object.
  *
  * @tfield int button The mouse button number, or 0 for any button.
--- a/objects/button.h
+++ b/objects/button.h
@@ -39,7 +39,7 @@ typedef struct button_t
     xcb_button_t button;
 } button_t;
 
-lua_class_t button_class;
+extern lua_class_t button_class;
 LUA_OBJECT_FUNCS(button_class, button_t, button)
 ARRAY_FUNCS(button_t *, button, DO_NOTHING)
 
--- a/objects/client.c
+++ b/objects/client.c
@@ -108,6 +108,8 @@
 #include <xcb/shape.h>
 #include <cairo-xcb.h>
 
+lua_class_t client_class;
+
 /** Client class.
  *
  * This table allow to add more dynamic properties to the clients. For example,
--- a/objects/client.h
+++ b/objects/client.h
@@ -200,7 +200,7 @@ struct client_t
 ARRAY_FUNCS(client_t *, client, DO_NOTHING)
 
 /** Client class */
-lua_class_t client_class;
+extern lua_class_t client_class;
 
 LUA_OBJECT_FUNCS(client_class, client_t, client)
 
--- a/objects/drawin.c
+++ b/objects/drawin.c
@@ -46,6 +46,8 @@
 #include <cairo-xcb.h>
 #include <xcb/shape.h>
 
+lua_class_t drawin_class;
+
 /** Drawin object.
  *
  * @field border_width Border width.
--- a/objects/drawin.h
+++ b/objects/drawin.h
@@ -52,7 +52,7 @@ void luaA_drawin_systray_kickout(lua_State *);
 
 void drawin_class_setup(lua_State *);
 
-lua_class_t drawin_class;
+extern lua_class_t drawin_class;
 
 #endif
 // vim: filetype=c:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80
--- a/objects/key.c
+++ b/objects/key.c
@@ -43,6 +43,8 @@
 #include <xkbcommon/xkbcommon.h>
 #include <glib.h>
 
+lua_class_t key_class;
+
 /** Key object.
  *
  * @tfield string key The key to trigger an event.
--- a/objects/key.h
+++ b/objects/key.h
@@ -36,7 +36,7 @@ typedef struct keyb_t
     xcb_keycode_t keycode;
 } keyb_t;
 
-lua_class_t key_class;
+extern lua_class_t key_class;
 LUA_OBJECT_FUNCS(key_class, keyb_t, key)
 DO_ARRAY(keyb_t *, key, DO_NOTHING)
 
--- a/objects/tag.c
+++ b/objects/tag.c
@@ -190,6 +190,8 @@
 #include "ewmh.h"
 #include "luaa.h"

+lua_class_t tag_class;
+
 /**
  * @signal request::select
  */
--- a/objects/tag.h
+++ b/objects/tag.h
@@ -46,7 +46,7 @@ struct tag
     client_array_t clients;
 };
 
-lua_class_t tag_class;
+extern lua_class_t tag_class;
 LUA_OBJECT_FUNCS(tag_class, tag_t, tag)
 
 void tag_class_setup(lua_State *);
--- a/objects/window.c
+++ b/objects/window.c
@@ -59,6 +59,7 @@
 #include "property.h"
 #include "xwindow.h"
 
+lua_class_t window_class;
 LUA_CLASS_FUNCS(window, window_class)
 
 static xcb_window_t
--- a/objects/window.h
+++ b/objects/window.h
@@ -80,7 +80,7 @@ typedef struct
     WINDOW_OBJECT_HEADER
 } window_t;
 
-lua_class_t window_class;
+extern lua_class_t window_class;
 
 void window_class_setup(lua_State *);