xerces: disable parallel builds

The build system has a race condition causing build failure with high
BR2_JLEVEL values, so ensure make -j1 is used.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/xerces/xerces.mk

@@ -78,7 +78,7 @@ $(XERCES_DIR)/.configured: $(XERCES_DIR)/.unpacked
 	touch $@
 
 $(XERCES_DIR)/src/.libs/$(LIBXERCES_BINARY): $(XERCES_DIR)/.configured
-	$(MAKE) $(TARGET_CONFIGURE_OPTS) $(XERCES_MAKE_OPT) -C $(XERCES_DIR)
+	$(MAKE1) $(TARGET_CONFIGURE_OPTS) $(XERCES_MAKE_OPT) -C $(XERCES_DIR)
 
 $(STAGING_DIR)/usr/lib/$(LIBXERCES_BINARY): $(XERCES_DIR)/src/.libs/$(LIBXERCES_BINARY)
 	$(MAKE) $(TARGET_CONFIGURE_OPTS) DESTDIR=$(STAGING_DIR) \