[autotools,bugfix] Adds share/dune/aclocal to the include path of autogen.

Previously aclocal did not find the DUNE macros when they were installed to
the new location. This patch fixes this by adding the path to this list of
include paths if it is available.
[cmake,release] Correctly treat multiarch triplets in Debian.

Debian uses multiarch-triplets (e.g. i386-gnu-linux) when installing
object libraries. These are installed to lib/$multiarch-triplet and not
to lib directly. In CMake this concept is partially supported by
GNUInstallDirs.cmake by setting CMAKE_INSTALL_LIBDIR to
lib/$multiarch-triplet. Unfortunately, this is only half the way through
as we also install libraries that do not differ between architectures (e.g.
our shell libraries.

This patch addes multiarch support for CMake to the best of my knowledge,
which might be limited, though:

- All not architecture dependent libraries are installed below the variable
  DUNE_INSTALL_NONOBJECTLIBDIR, which defaults to lib, but might be adjusted
  by the user.
- If the module ships an object library then the pkg-config and CMake package
  configuration files are installed in lib/$multiarch-triplet/pkg-config and
  lib/$multiarch-triplet/cmake/modules, respectively.
- If not then they are installed in lib/pkg-config and
  lib/cmake/modules, respectively.

Note that for autotool no such difference exists and always the latter approach
is used.