ccdpack: Fix some pointer comparisons

It seems that on my Mac (at least):

    if ( ++c >= retbuf + BUFLENG ) {

comes out true even when `c` and `retbuf` differ by 1 with `c`
less than `retbuf`. It looks like this is some issues with
64-bit unsigned types and signed ints. Recasting it explicitly
to use a ptrdiff_t type fixes the problem:

    c++;
    if ( (ptrdiff_t)(c - retbuf) >= BUFLENG ) {

Before this fix pairndf was failing even before it started with
a buffer overflow in the Tcl communication system.

After this patch pairndf at least throws up a couple of images
although I'm not sure what I'm supposed to do with them once
they are up.

applications/ccdpack/ccdwish.c

@@ -91,6 +91,8 @@
 #include <string.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <stddef.h>
+
 #include "tcl.h"
 #include "tk.h"
 #include "mers.h"
@@ -422,7 +424,8 @@ extern F77_SUBROUTINE(ccd1_linflt)();
          c = buffer - 1;
          do {
             int bytes;
-            if ( ++c >= buffer + BUFLENG ) {
+            c++;
+            if (  (ptrdiff_t)(c - buffer) >= BUFLENG ) {
                strcpy( buffer, "Buffer overflow in Tcl process\n" );
                write( ofd, buffer, strlen( buffer ) );
                exit( 1 );

applications/ccdpack/main/tcltalk.c

@@ -81,7 +81,8 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <signal.h>
-#include <errno.h>
+#include <stddef.h>
+
 #include "tcl.h"
 #include "sae_par.h"
 #include "tcltalk.h"
@@ -369,7 +370,8 @@
          c = retbuf - 1;
          *retbuf = '\0';
          do {
-            if ( ++c >= retbuf + BUFLENG ) {
+            c++;
+            if ( (ptrdiff_t)(c - retbuf) >= BUFLENG ) {
                *status = SAI__ERROR;
                errRep( "CCD_TCL_BUF", "Buffer overflow", status );
                return NULL;