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/objects.c

@@ -876,7 +876,7 @@ SCROLL(None, "STRC PRST SKRZ KRK",  1,   0, 100), /* Czech and Slovak
                                                         tongue-twister */
     /* These must come last because they have special fixed descriptions.
      */
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
 SCROLL("mail",          "stamped",  0,   0,   0),
 #endif
 SCROLL("blank paper", "unlabeled",  0,  28,  60),