[release] consistently use std::multiplies in CollectiveCommunication::prod

Somehow, the vector version of CollectiveCommunication< MPI_Comm >::prod
passed std::plus to allreduce (instead of std::multiplies). This patch
corrects this flaw.
(cherry picked from commit 58e4d8a393d09dbc5f05e9d3fd6fe491616068a3)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[buildsystem, release] Add CMake package configuration files with autotools.
(cherry picked from commit edf71b860a6ad18bfdfff2e8d74fd53975820322)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[cmake,release] Remove debugging output
(cherry picked from commit 0ad82993cd94d504f387feb45ab7835e6ccbb701)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[bugfix] make compile when DUNE_ISTL_WITH_CHECKING is defined.
(cherry picked from commit 396f32fb04e1972a4bb2b7ba951c59709fd7cc9b)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>