separate MAIL functionality from MAIL-related structure inclusion

With 3.7+ aspirations of improving savefile interoperability between 32-bit
and 64-bit builds, as well as between platforms, it is better to not have
the underlying struct/array content be conditional.

This splits off some of the MAIL code into MAIL_STRUCTURES code. In theory,
since MAIL_STRUCTURES is unconditionally included, the macro could
just go away and leave that code unconditional, but this commit doesn't
go that far.

src/role.c

@@ -2084,7 +2084,7 @@ struct monst *mtmp;
         return "Aloha"; /* Hawaiian */
     case PM_VALKYRIE:
         return
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
                (mtmp && mtmp->data == &mons[PM_MAIL_DAEMON]) ? "Hallo" :
 #endif
                "Velkommen"; /* Norse */