gh-129539: Include sysexits.h before checking EX_OK

Previously the macro would be redefined when the header was included.

Author: collinfunk

Misc/NEWS.d/next/Build/2025-01-31-19-01-24.gh-issue-129539.6ndioH.rst

@@ -0,0 +1 @@
+Don't redefine ``EX_OK`` when the system has the ``sysexits.h`` header.

Modules/posixmodule.c

@@ -52,10 +52,6 @@
 #  include "winreparse.h"
 #endif
 
-#if !defined(EX_OK) && defined(EXIT_SUCCESS)
-#  define EX_OK EXIT_SUCCESS
-#endif
-
 #ifdef __APPLE__
  /* Needed for the implementation of os.statvfs */
 #  include <sys/param.h>
@@ -292,7 +288,11 @@ corresponding Unix manual entries for more information on calls.");
 #endif
 
 #ifdef HAVE_SYSEXITS_H
-#  include <sysexits.h>
+#  include <sysexits.h>           //  EX_OK
+#endif
+
+#if !defined(EX_OK) && defined(EXIT_SUCCESS)
+#  define EX_OK EXIT_SUCCESS
 #endif
 
 #ifdef HAVE_SYS_LOADAVG_H