Fix Issue dankogai#129

PERL_UNUSED_VAR doesn't work for variables marked 'const' for perls
earlier than about 5.9.3.  This commit replaces the failing call with
the modern expansion of it.  This should suffice for now, as this code
is planned to be replaced in the next few months.

Encode.xs

@@ -533,7 +533,7 @@ process_utf8(pTHX_ SV* dst, U8* s, U8* e, SV *check_sv,
 
 #else   /* Use code for earlier perls */
 
-        PERL_UNUSED_VAR(flags);
+        ((void)sizeof(flags));  /* Avoid compiler warning */
 
         if (UTF8_IS_INVARIANT(*s)) {
             *d++ = *s++;