diff --git a/mozilla/js/src/Makefile.in b/mozilla/js/src/Makefile.in
index 79b9eca7390..1198200e3e4 100644
--- a/mozilla/js/src/Makefile.in
+++ b/mozilla/js/src/Makefile.in
@@ -373,14 +373,32 @@ ifdef HOST_NSPR_MDCPUCFG
 HOST_CC := $(HOST_CC) -DMDCPUCFG=$(TARGET_NSPR_MDCPUCFG)
 endif
 
+ifdef CROSS_COMPILE
+# jscpucfg needs to know when it's supposed to produce a config for the target
+JSCPUCFG_DEFINES = -DCROSS_COMPILE=1
+
+# This is incredibly hacky.  Darwin NSPR uses the same MDCPUCFG for multiple
+# processors, and determines which processor to configure for based on
+# #ifdef i386.  This macro is among the NSPR defines, but is also automatically
+# defined by the compiler when building for i386.  It therefore needs to be
+# defined here if targeting i386, and explicitly undefined otherwise.
+ifeq ($(OS_ARCH),Darwin)
+ifeq ($(TARGET_CPU),powerpc)
+DEFINES += -Ui386
+else
+DEFINES += -Di386=1
+endif
+endif
+endif
+
 ifeq ($(OS_ARCH),QNX)
 ifneq ($(OS_TARGET),NTO)
 # QNX's compiler apparently can't build a binary directly from a source file.
 jscpucfg.o: jscpucfg.c Makefile.in
-	$(HOST_CC) $(HOST_CFLAGS) -c $(DEFINES) $(NSPR_CFLAGS) -o $@ $<
+	$(HOST_CC) $(HOST_CFLAGS) -c $(JSCPUCFG_DEFINES) $(DEFINES) $(NSPR_CFLAGS) -o $@ $<
 
 jscpucfg: jscpucfg.o
-	$(HOST_CC) $(HOST_CFLAGS) $(DEFINES) -o $@ $<
+	$(HOST_CC) $(HOST_CFLAGS) $(JSCPUCFG_DEFINES) $(DEFINES) -o $@ $<
 endif
 else
 ifeq ($(OS_ARCH),WINCE)
@@ -388,7 +406,7 @@ jscpucfg$(HOST_BIN_SUFFIX):
 	echo no need to build jscpucfg $<
 else
 jscpucfg$(HOST_BIN_SUFFIX): jscpucfg.c Makefile.in
-	$(HOST_CC) $(HOST_CFLAGS) $(DEFINES) $(NSPR_CFLAGS) $(OUTOPTION)$@ $<
+	$(HOST_CC) $(HOST_CFLAGS) $(JSCPUCFG_DEFINES) $(DEFINES) $(NSPR_CFLAGS) $(OUTOPTION)$@ $<
 endif
 endif