#9862: On AIX PIPE_BUF is broken.  Make it 512.

Patch by Sébastien Sablé.
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 5c475f5..58cc4c7 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -1786,6 +1786,10 @@
     PyModule_AddObject(m, "error", SelectError);
 
 #ifdef PIPE_BUF
+#ifdef HAVE_BROKEN_PIPE_BUF
+#undef PIPE_BUF
+#define PIPE_BUF 512
+#endif
     PyModule_AddIntConstant(m, "PIPE_BUF", PIPE_BUF);
 #endif