Use supported case changing macros

The code as-is no longer compiles due to changes in the API required
because of security issues.  However this patch causes it to compile
back to 5.6.0 as long as ppport.h version 3.55 or later is included.

This fix is part of Perl/perl5#17024

encGlue.c

@@ -273,7 +273,7 @@ Tcl_UtfToLower (char * src)
  while (*s)
   {
    STRLEN len;
-   Perl_to_utf8_lower(aTHX_ s, d, &len );
+   toLOWER_utf8_safe(s, s + UTF8_CHK_SKIP(s), d, &len );
    d += len;
    s += len;
   }
@@ -290,7 +290,7 @@ Tcl_UtfToUpper(char * src)
  while (*s)
   {
    STRLEN len;
-   Perl_to_utf8_upper(aTHX_ s, d, &len );
+   toUPPER_utf8_safe(s, s + UTF8_CHK_SKIP(s), d, &len );
    d += len;
    s += len;
   }