#4 the doc makefile gets a double DESTDIR

[R.J.V. Bertin]

2 items, grop.desktop and grop.png are installed in the wrong location by doc/Makefile because the $(DESTDIR) variable is expanded twice. This goes undetected when that variable is unused, but gives interesting effects when it is.

I'm attaching a patch.

[TIM]

This one works for me.