[bugfix] Use exter "C" to fix linker errors wit scotch.

Unlike the original METIS/ParMETIS scotch does not use extern "C"
in its headers. This lead to linker errors as descibed in
http://lists.dune-project.org/pipermail/dune/2014-March/012745.html.
This patch fixes this by guarding the includes with an extra extern "C"
directive.