PATCH: Support DESTDIR

[Andrew Cagney]

Hmm, which of these is correct:

> -	$(SHELL) $(srcdir)/../../mkinstalldirs $(infodir)
> +	$(SHELL) $(srcdir)/../../mkinstalldirs $(DESTDIR)/$(infodir)

> -		$(INSTALL_DATA) $$i $(htmldir)/$$i ; \
> +		$(INSTALL_DATA) $$i $(DESTDIR)$(htmldir)/$$i ; \

(the slash)?

 > +	$(mkinstalldirs) $(DESTDIR)$(infodir)

Please keep the mkinstalldirs consistent with the rest of GDB vis:
	$(SHELL) $(srcdir)/../mkinstalldirs $(DESTDIR)$infodir)
The addition of mkinstalldirs should be mentioned in the ChangeLog.

Andrew