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

Where 'weird' is defined as meaning something that the normal rules
don't apply, so something we generate is unlikely to be correct.

This only affects one element, and it uses aTHX in a way that is
incompatible with it being automated.

Author: khwilliamson

proto.h

@@ -188,6 +188,9 @@ sub generate_proto_h {
             # Don't generate a prototype for a macro that is not usable by the
             # outside world.
             next unless $flags =~ /[ACE]/;
+
+            # Nor one that is weird, which would likely be a syntax error.
+            next if $flags =~ /u/;
         }
         else {
             die_at_end "$plain_func: u flag only usable with m" if $flags =~ /u/;