diff --git a/src/ChangeLog b/src/ChangeLog
index 56a678957c8..84224e57ad0 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,9 @@
+2000-06-22  Dave Love  <fx@gnu.org>
+
+	* s/isc3-0.h (C_SWITCH_SYSTEM): Define _XOPEN_SOURCE=500.
+
+	* s/gnu-linux.h (C_SWITCH_SYSTEM): Don't define _XOPEN_SOURCE here.
+
 2000-06-22  Kenichi Handa  <handa@etl.go.jp>
 
 	* ccl.c (ccl_driver) <CCL_End>: Decrement stack_idx only when it
diff --git a/src/s/isc3-0.h b/src/s/isc3-0.h
index 737ce3c3297..be1181ae5d1 100644
--- a/src/s/isc3-0.h
+++ b/src/s/isc3-0.h
@@ -34,8 +34,9 @@
 /* We need either _XOPEN_SOURCE or _POSIX_SOURCE to import the posix
    signal symbols; might as well use _XOPEN_SOURCE.  Defining _SYSV3
    ensures that we don't lose the traditional symbols as a side effect
-   from this or __STDC__ being defined.  */
-#define C_SWITCH_SYSTEM -D_XOPEN_SOURCE -D_SYSV3
+   from this or __STDC__ being defined.  Define _XOPEN_SOURCE=500 for
+   compatibility with the autoconf test.  */
+#define C_SWITCH_SYSTEM -D_XOPEN_SOURCE=500 -D_SYSV3
 
 #ifdef __GNUC__  /* Currently we use -lcposix only with gcc */
 /* This works around a bug in ISC 4.0 and 3.0; it fails