Avoid blocking "clean" on changes

Makefile

@@ -817,19 +817,19 @@ clean:		common_clean handler_clean portal_clean manager_clean omegat-clean stop_
 	@echo "Cleaned"
 
 common_clean:
-	@if test -e ${SRCCOMMONDIR}/Makefile;then $(MAKE) -C ${SRCCOMMONDIR} distclean;fi
+	-@if test -e ${SRCCOMMONDIR}/Makefile;then $(MAKE) -C ${SRCCOMMONDIR} distclean;fi
 	@rm -vf common*
 
 handler_clean:
-	@if test -e ${SRCHANDLERDIR}/Makefile;then $(MAKE) -C ${SRCHANDLERDIR} distclean;fi
+	-@if test -e ${SRCHANDLERDIR}/Makefile;then $(MAKE) -C ${SRCHANDLERDIR} distclean;fi
 	@rm -vf handler*
 
 portal_clean:
-	@if test -e ${SRCPORTALDIR}/Makefile;then $(MAKE) -C ${SRCPORTALDIR} distclean;fi
+	-@if test -e ${SRCPORTALDIR}/Makefile;then $(MAKE) -C ${SRCPORTALDIR} distclean;fi
 	@rm -vf portal*
 
 manager_clean:
-	@if test -e ${SRCMANAGERDIR}/Makefile;then $(MAKE) -C ${SRCMANAGERDIR} distclean;fi
+	-@if test -e ${SRCMANAGERDIR}/Makefile;then $(MAKE) -C ${SRCMANAGERDIR} distclean;fi
 	@rm -vf manager*
 
 # Perl libraries uninstall targets