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/mail.c

@@ -10,8 +10,12 @@
 # include <fcntl.h>
 # include <errno.h>
 #endif /* SIMPLE_MAIL */
+#endif /* MAIL */
+#ifdef MAIL_STRUCTURES
 #include "mail.h"
+#endif
 
+#ifdef MAIL
 /*
  * Notify user when new mail has arrived.  Idea by Merlyn Leroy.
  *