Fix macOS printing error message issue.

Previously Tux Paint was raising an error informing the user it could not
print even when it did print successfully.  This appears to be due to our
lpr printing routine's assumption that errno is reset on success whereas
the C library in macOS does not appear to do so.  To address this issue,
errno is reset before calling pclose() (on macOS and macOS only - just in
case this code has an unintended side-effect on other platforms).  This
code change assumes we're only interested in errno set by pclose(), and
that our lpr printing routine is not interested in errno raised prior to
pclose().

src/postscript_print.c

@@ -3,7 +3,7 @@
 
   For Tux Paint
   PostScript(r) printing routine.
-  (for non-Windows, non-Mac OS X, non-BeOS platforms, e.g. Linux)
+  (for non-Windows, non-BeOS platforms, e.g. Linux and macOS)
   (moved from tuxpaint.c in 0.9.17)
 
   Copyright (c) 2009 by Bill Kendrick and others
@@ -51,6 +51,7 @@
 #include <paper.h>
 #include <math.h>
 #include <errno.h>
+#include "debug.h"
 
 #ifndef PAPER_H
 #error "---------------------------------------------------"
@@ -296,13 +297,18 @@ int do_ps_save(FILE * fi,
       pid_t child_pid, w;
       int status;
 
+#ifdef __APPLE__
+      /* macOS does not always reset errno so Tux Paint thinks print never
+       * succeeds - let's reset before calling pclose() on macOS */
+      errno = 0;
+#endif
+
       child_pid = pclose(fi);
 
-/* debug */
-/*
+#ifdef DEBUG
     printf("pclose returned %d\n", child_pid); fflush(stdout);
     printf("errno = %d\n", errno); fflush(stdout);
-*/
+#endif
 
       if (child_pid < 0 || (errno != 0 && errno != EAGAIN))
         {                       /* FIXME: This right? */
@@ -317,8 +323,7 @@ int do_ps_save(FILE * fi,
         {
           w = waitpid(child_pid, &status, 0);
 
-/* debug */
-/*
+#ifdef DEBUG
             if (w == -1) { perror("waitpid"); exit(EXIT_FAILURE); }
             if (WIFEXITED(status)) {
                 printf("exited, status=%d\n", WEXITSTATUS(status));
@@ -329,7 +334,7 @@ int do_ps_save(FILE * fi,
             } else if (WIFCONTINUED(status)) {
                 printf("continued\n");
             }
-*/
+#endif
         }
       while (w != -1 && !WIFEXITED(status) && !WIFSIGNALED(status));