spawn: Fix compilation error on macOS

[Bruno Haible]

On macOS 12.5, in a testdir for module 'execute', I'm seeing these
compilation errors:

In file included from ../../gllib/execute.c:71:
./spawn.h:692:19: error: use of undeclared identifier 
'POSIX_SPAWN_SETSCHEDPARAM'
                | POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER)
                  ^
./spawn.h:692:47: error: use of undeclared identifier 'POSIX_SPAWN_SETSCHEDULER'
                | POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER)
                                              ^

This patch fixes them.


2023-02-04  Bruno Haible  <bruno@clisp.org>

        spawn: Fix compilation error on macOS (regression 2023-01-22).
        * lib/spawn.in.h: Tweak logic for defining the POSIX_SPAWN_* constants.

diff --git a/lib/spawn.in.h b/lib/spawn.in.h
index 958464a7d2..b35e5d77b3 100644
--- a/lib/spawn.in.h
+++ b/lib/spawn.in.h
@@ -144,19 +144,18 @@ typedef struct
 
 /* Flags to be set in the 'posix_spawnattr_t'.  */
 #if @HAVE_POSIX_SPAWN@
-# if !@REPLACE_POSIX_SPAWN@
-/* Use the values from the system, but provide the missing ones.  */
-#  ifndef POSIX_SPAWN_SETSCHEDPARAM
-#   define POSIX_SPAWN_SETSCHEDPARAM 0
-#  endif
-#  ifndef POSIX_SPAWN_SETSCHEDULER
-#   define POSIX_SPAWN_SETSCHEDULER 0
-#  endif
-# else /* @REPLACE_POSIX_SPAWN@ */
+# if @REPLACE_POSIX_SPAWN@
 /* Use the values from the system, for better compatibility.  */
 /* But this implementation does not support AIX extensions.  */
 #   undef POSIX_SPAWN_FORK_HANDLERS
 # endif
+/* Provide the values that the system is lacking.  */
+# ifndef POSIX_SPAWN_SETSCHEDPARAM
+#  define POSIX_SPAWN_SETSCHEDPARAM 0
+# endif
+# ifndef POSIX_SPAWN_SETSCHEDULER
+#  define POSIX_SPAWN_SETSCHEDULER 0
+# endif
 #else /* !@HAVE_POSIX_SPAWN@ */
 # define POSIX_SPAWN_RESETIDS           0x01
 # define POSIX_SPAWN_SETPGROUP          0x02