This is easy to do and can be useful when using an atomic initialization
counter.
The only platform where not returning the new value might be more
efficient than returning it is Solaris which is not really relevant any
more and on all the other platforms the functions we were already using
provided this value already (or almost, in case of using gcc builtins,
when we just have to use a different one).
Assigning __sync_sub_and_fetch() result to an unused variable can result in it
being optimized away and the function support is then not really tested for,
see #4542. So do make sure this variable is kept by making it volatile.
Closes#15555.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74934 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
