Fix -fno-common build failure. Taken from
https://github.com/awesomeWM/awesome/commit/d256d9055095f27a33696e0aeda4ee20ed4fb1a0

Index: luaa.c
--- luaa.c.orig
+++ luaa.c
@@ -77,6 +77,8 @@ extern const struct luaL_Reg awesome_root_lib[];
 extern const struct luaL_Reg awesome_mouse_methods[];
 extern const struct luaL_Reg awesome_mouse_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},