Test for the dirfd library function (for use in some upcoming
changes).
diff --git a/pyconfig.h.in b/pyconfig.h.in
index a2bb1cd..7a20810 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -178,6 +178,9 @@
    */
 #undef HAVE_DIRENT_H
 
+/* Define to 1 if you have the `dirfd' function. */
+#undef HAVE_DIRFD
+
 /* Define to 1 if you have the <dlfcn.h> header file. */
 #undef HAVE_DLFCN_H