Small changes in casts from void* to functions

Macro moved to llimits.h, and casts from void* to lua_CFunction first
go through 'voidf' (a pointer to a function from void to void), a kind
of void* for functions.

llimits.h

@@ -152,6 +152,26 @@ typedef LUAI_UACINT l_uacInt;
 #endif
 
 
+/*
+** Special type equivalent to '(void*)' for functions (to suppress some
+** warnings when converting function pointers)
+*/
+typedef void (*voidf)(void);
+
+
+/*
+** Macro to convert pointer-to-void* to pointer-to-function. This cast
+** is undefined according to ISO C, but POSIX assumes that it works.
+** (The '__extension__' in gnu compilers is only to avoid warnings.)
+*/
+#if defined(__GNUC__)
+#define cast_func(p) (__extension__ (voidf)(p))
+#else
+#define cast_func(p) ((voidf)(p))
+#endif
+
+
+
 /*
 ** non-return type
 */