Put DES diagnostics behind an ifdef of their own.

I think Pavel is right to have turned off -DDEBUG in the MinGW build
on general principles - it should never be the default option for any
build platform - but also, it was not intentional that sshdes.c
produces its hugely detailed diagnostics merely because you compile
with the very generic -DDEBUG. So now you have to say
-DDES_DIAGNOSTICS too if you really want sshdes.c's gory detail.

sshdes.c

@@ -61,6 +61,14 @@
 #include "ssh.h"
 #include "mpint_i.h"               /* we reuse the BignumInt system */
 
+/* If you compile with -DDES_DIAGNOSTICS, intermediate results will be
+ * sent to debug() (so you also need to compile with -DDEBUG).
+ * Otherwise this ifdef will condition away all the debug() calls. */
+#ifndef DES_DIAGNOSTICS
+#undef debug
+#define debug(...) ((void)0)
+#endif
+
 /*
  * General utility functions.
  */