Fix rules for parallel make.

Index: modules/syscalls/Makefile.in
--- modules/syscalls/Makefile.in.orig
+++ modules/syscalls/Makefile.in
@@ -47,7 +47,7 @@ preload.lisp : $(srcdir)/preload.lisp
 am--refresh :
 
 # syscalls relies on src/gllib, so we must also include files in the build dir
-gllib libgnu_sc.a : force
+gllib libgnu_sc.a &: force
 	rm -f libgnu_sc.a
 	mkdir -p gllib
 	test -f gllib/Makefile || sh config.status gllib/Makefile depfiles