Previously we modified ALBERTA_LIBS in configure.ac as some of the variables
used there are only set in downstream modules. In addition there was a check
whether dune-grid is installed during setting the alberta libraries in
m4/alberta.m4. Unfortunately this never worked as the variables were not
yet set at this point.

Now we simply set DUNE_GRID_LIBDIR in configure.ac and use the same stanza for
setting the alberta libs
ALBERTA${N}D_LIBS="'-L\$(DUNE_GRID_LIBDIR) -ldunealbertagrid_${N}d -ldunegrid \$(ALBERTA_LIBPATHFLAGS) -lalberta_${N}d \$(ALBERTA_BASE_LIBS)'")
no matter whether dune-grid is installed or not and rely on libtool to do
the magic.

Fixes flyspray 1414 https://dune-project.org/flyspray/index.php?do=details&task_id=1414
as confirmed by Ansgar. Kudos to him for testing.