all SCHED_ constants are optional (closes #27656)

Misc/NEWS

@@ -34,6 +34,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #27656: Do not assume sched.h defines any SCHED_* constants.
+
 - Issue #27130: In the "zlib" module, fix handling of large buffers
   (typically 4 GiB) when compressing and decompressing.  Previously, inputs
   were limited to 4 GiB, and compression and decompression operations did not

Modules/posixmodule.c

@@ -12750,9 +12750,15 @@ all_ins(PyObject *m)
 #endif
 
 #ifdef HAVE_SCHED_H
+#ifdef SCHED_OTHER
     if (PyModule_AddIntMacro(m, SCHED_OTHER)) return -1;
+#endif
+#ifdef SCHED_FIFO
     if (PyModule_AddIntMacro(m, SCHED_FIFO)) return -1;
+#endif
+#ifdef SCHED_RR
     if (PyModule_AddIntMacro(m, SCHED_RR)) return -1;
+#endif
 #ifdef SCHED_SPORADIC
     if (PyModule_AddIntMacro(m, SCHED_SPORADIC) return -1;
 #endif