Avoid C++20 deprecated assignment to volatile

794c1a30bc "macro wrappers for
g_once_init_enter/leave" added this line (whose intent is unclear to me).

<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1152r4.html>
"Deprecating volatile" (scheduled for inclusion in C++20) will make the
assignment expression

  *(location) = (result)

deprecated when the LHS is of (non-class) volatile type, which is the case when
g_once_init_leave is expanded as part of e.g. G_DEFINE_TYPE_WITH_CODE (in
gobject/gtype.h), where location is a pointer to some

  static volatile gsize g_define_type_id__volatile = 0;

Recent Clang trunk emits -Wdeprecated-volatile for it under -std=c++2a since
<https://github.com/llvm/llvm-project/commit/
4a6861a7e5b59be24a09b8b9782255d028e7aade> "[c++20] P1152R4: warn on any
simple-assignment to a volatile lvalue".

The fix is to make the assignment expression a discared-value expression by
casting it to void (which in turn requires casting the second branch of the
surrounding conditional expression to void, too; not sure what the top-level
cast to void was intended for, and whether it would still be needed under
certain circumstances).

glib/gthread.h

@@ -254,7 +254,7 @@ void            g_once_init_leave               (volatile void  *location,
 # define g_once_init_leave(location, result) \
   (G_GNUC_EXTENSION ({                                               \
     G_STATIC_ASSERT (sizeof *(location) == sizeof (gpointer));       \
-    (void) (0 ? *(location) = (result) : 0);                         \
+    0 ? (void) (*(location) = (result)) : (void) 0;                  \
     g_once_init_leave ((location), (gsize) (result));                \
   }))
 #else