[cmake] Export HAVE_DUNE_ISTL for other modules.

This is e.g. needed in the parallel version of autodiff to
test whether ISTL is there.
Removed superfluous export of HAVE_DUNE_ISTL.

With the last patch HAVE_DUNE_ISTL will already be
defined in config.h
Throw an error when using a sequential solver in parallel
[bugfix] Use the size of the vector for the copying.

The last patch did not compile as there was no size member
in scope. Therefore this patch resorts to using the size of
the vector.