use make variable to avoid QA issue: "make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule"

--- a/GNUmakefile
+++ b/GNUmakefile
@@ -576,7 +576,7 @@ doc/home-page.html: doc/home-page.lisp
 # xdoc::save that populates doc/manual/ (not under books/).
 acl2-manual: check-books
 	rm -rf doc/manual books/system/doc/acl2-manual.cert
-	cd books ; make USE_QUICKLISP=1 system/doc/acl2-manual.cert
+	cd books ; $(MAKE) USE_QUICKLISP=1 system/doc/acl2-manual.cert
 	rm -rf doc/manual/download/*
 
 # WARNING: The dependency list just below isn't complete, since it