[cmake][bugfix] Exclude hidden headers from header checks.

Previously hidden unix files were included in the headercheck.
Unfortunately  emacs(server) tends to produces files such as
.#<headername>. This broke the constructed Makefiles. This patch
excludes such files from the list and fixes the issue.
[Bugfix] Add new header visiblity.hh to HEADERS/ installed files.

Thanks to Tobias Malkmus for the heads-up.
[MPIHelper] Add DUNE_EXPORT hints to singleton accessors of MPIHelper
Merge remote-tracking branch 'origin/master' into feature/FS1191-assigner
[static assertion] Make more precise statement to the user in static assertion.