Make the syntax for compound command lines more portable (nmake, CygWin).

This is probably not sufficient by itself for the info generation, and is
being done mostly to keep the style consistent with additional patches
that will be applied to Doc/Makefile.

Doc/info/Makefile

@@ -49,25 +49,25 @@ clobber: clean
 # in case we haven't already built the HTML:
 
 $(HTMLDIR)/api/api.html:
-	(cd $(HTMLDIR); $(MAKE) api)
+	cd $(HTMLDIR) && $(MAKE) api
 
 $(HTMLDIR)/ext/ext.html:
-	(cd $(HTMLDIR); $(MAKE) ext)
+	cd $(HTMLDIR) && $(MAKE) ext
 
 $(HTMLDIR)/lib/lib.html:
-	(cd $(HTMLDIR); $(MAKE) lib)
+	cd $(HTMLDIR) && $(MAKE) lib
 
 $(HTMLDIR)/mac/mac.html:
-	(cd $(HTMLDIR); $(MAKE) mac)
+	cd $(HTMLDIR) && $(MAKE) mac
 
 $(HTMLDIR)/ref/ref.html:
-	(cd $(HTMLDIR); $(MAKE) ref)
+	cd $(HTMLDIR) && $(MAKE) ref
 
 $(HTMLDIR)/tut/tut.html:
-	(cd $(HTMLDIR); $(MAKE) tut)
+	cd $(HTMLDIR) && $(MAKE) tut
 
 $(HTMLDIR)/dist/dist.html:
-	(cd $(HTMLDIR); $(MAKE) dist)
+	cd $(HTMLDIR) && $(MAKE) dist
 
 $(HTMLDIR)/inst/inst.html:
-	(cd $(HTMLDIR); $(MAKE) inst)
+	cd $(HTMLDIR) && $(MAKE) inst