regen/embed.pl: Don't create prototype for private macros

Prior to this commit, a commented-out prototype was created for all
macros that we have argument and return type information for.  This
might be useful information for a reader of proto.h.

This commits stops doing this for private macros.  It makes a future
commit slightly easier, and I'm unsure of the usefulness of this anyway.
But I could be persuaded otherwise.

Author: khwilliamson

proto.h

@@ -191,6 +191,10 @@ sub generate_proto_h {
                 die_at_end
                           "$plain_func: m and S flags are mutually exclusive";
             }
+
+            # Don't generate a prototype for a macro that is not usable by the
+            # outside world.
+            next unless $flags =~ /[ACE]/;
         }
         else {
             die_at_end "$plain_func: u flag only usable with m"