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.

win/share/tilemap.c

@@ -54,7 +54,7 @@ struct conditionals {
 #ifndef CHARON /* not supported yet */
     { MON_GLYPH, PM_CROESUS, "Charon" },
 #endif
-#ifndef MAIL
+#ifndef MAIL_STRUCTURES
     { MON_GLYPH, PM_FAMINE, "mail daemon" },
 #endif
     /* commented out in monst.c at present */
@@ -69,7 +69,7 @@ struct conditionals {
 /* allow slime mold to look like slice of pizza, since we
  * don't know what a slime mold should look like when renamed anyway
  */
-#ifndef MAIL
+#ifndef MAIL_STRUCTURES
     { OBJ_GLYPH, SCR_STINKING_CLOUD + EXTRA_SCROLL_DESCR_COUNT,
       "stamped / mail" },
 #endif