Fix rules for parallel make.

Index: modules/regexp/Makefile.in
--- modules/regexp/Makefile.in.orig
+++ modules/regexp/Makefile.in
@@ -44,7 +44,7 @@ preload.lisp : $(srcdir)/preload.lisp
 am--refresh :
 
 # regexp relies on src/gllib, so we must also include files in the build dir
-gllib libgnu_rx.a : force
+gllib libgnu_rx.a &: force
 	rm -f libgnu_rx.a
 	mkdir -p gllib
 	test -f gllib/Makefile || sh config.status gllib/Makefile depfiles