Mark a few functions as __attribute__((noreturn)).

This is mostly to make static analysers and compiler warnings a bit
happier - now they know that a call to, say, modalfatalbox() means
they don't have to worry about what the rest of the function will do.

defs.h

@@ -113,4 +113,10 @@ typedef struct PacketProtocolLayer PacketProtocolLayer;
     TYPECHECK(object == &((type *)0)->field,                            \
               ((type *)(((char *)(object)) - offsetof(type, field))))
 
+#if defined __GNUC__ || defined __clang__
+#define NORETURN __attribute__((__noreturn__))
+#else
+#define NORETURN
+#endif
+
 #endif /* PUTTY_DEFS_H */

pscp.c

@@ -226,7 +226,7 @@ static void ssh_scp_init(void)
 /*
  *  Print an error message and exit after closing the SSH link.
  */
-static void bump(const char *fmt, ...)
+static NORETURN void bump(const char *fmt, ...)
 {
     char *str, *str2;
     va_list ap;

putty.h

@@ -1141,11 +1141,8 @@ struct TermWinVtable {
  * Global functions not specific to a connection instance.
  */
 void nonfatal(const char *, ...);
-void modalfatalbox(const char *, ...);
-#ifdef macintosh
-#pragma noreturn(modalfatalbox)
-#endif
-void cleanup_exit(int);
+NORETURN void modalfatalbox(const char *, ...);
+NORETURN void cleanup_exit(int);
 
 /*
  * Exports from conf.c, and a big enum (via parametric macro) of