diff --git a/src/ecmascript/spidermonkey-shared.h b/src/ecmascript/spidermonkey-shared.h
index 98bfe311c..8154ea8e2 100644
--- a/src/ecmascript/spidermonkey-shared.h
+++ b/src/ecmascript/spidermonkey-shared.h
@@ -1,17 +1,22 @@
 #ifndef EL__ECMASCRIPT_SPIDERMONKEY_SHARED_H
 #define EL__ECMASCRIPT_SPIDERMONKEY_SHARED_H
 
-/* For wild SpiderMonkey installations. */
+/* Tell SpiderMonkey headers which operating system will be used.
+ * With standalone SpiderMonkey 1.8.5, "pkg-config --cflags mozjs185"
+ * does not define such a macro, so ELinks must do that here.
+ * With xulrunner 2.0 however, "pkg-config --cflags mozilla-js"
+ * already outputs -DXP_UNIX or similar.  To prevent a warning about
+ * macro redefinition, define the macro as 1, like -D does.  */
 #ifdef CONFIG_OS_BEOS
-#define XP_BEOS
+#define XP_BEOS 1
 #elif CONFIG_OS_OS2
-#define XP_OS2
+#define XP_OS2 1
 #elif CONFIG_OS_RISCOS
 #error Out of luck, buddy!
 #elif CONFIG_OS_UNIX
-#define XP_UNIX
+#define XP_UNIX 1
 #elif CONFIG_OS_WIN32
-#define XP_WIN
+#define XP_WIN 1
 #endif
 
 #include <jsapi.h>