glib/goption.c: Fix build on MSVC

Use #ifdef rather than #if, as MSVC does not like #if <macro> without
parens, and this is the normal usage in other cases like this.
gio: Add missing transfer annotation to g_simple_proxy_resolver_new
gobject: Add missing transfer annotation to g_boxed_copy
Drop a no-longer-existing example from the docs

gapplication-example-menu.c was dropped in
0c094d660769a00564ef33a775a387f62cf2ff41, two years ago.
Time to remove its inclusion in the docs too.

https://bugzilla.gnome.org/show_bug.cgi?id=722973
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