[Dune] AC_LIB_PROG_LD_GNU in grape.m4

[Christian Engwer]

Well Robert,

> I added this check since some gnu linker flags are added later which
> should only be done for the gnu linker of course. If the AC_PROG_LD_GNU?
> does the same then please replace this.

that was exactly my question. At the first view the two functions are
supposed to do the same, but I don't know whether the grape test might
rely on some special things done inside the method which are not
available in the other. What was the reason to choose this method over
the alternative one?

Thanks
  Christian