Merge branch 'bugfix/fix-build-with-clang-6' into 'master'

clang 6 hangs indefinitely when trying to compile gitter_mgb.cc or
gitter_pll_mgb.cc with an optimization level of -O2 or higher. I tried
isolating the exact cause of the problem, but didn't have much luck with that,
so this patch adds some CMake logic that detects the problematic case and
forces the optimization level for those two files down to -O1 if necessary,
but avoids bumping it up in case of a debug build.

If possible, it would be good to also backport this patch to the 2.6 release
branch.

See merge request [extensions/dune-alugrid!49]

  [extensions/dune-alugrid!49]: gitlab.dune-project.org/extensions/dune-alugrid/merge_requests/49

[cmake] Link libdunealugrid against DUNE_LIBS

See merge request extensions/dune-alugrid!9

clang 6 hangs indefinitely when trying to compile gitter_mgb.cc or gitter_pll_mgb.cc with an
optimization level of -O2 or higher. This patch adds some CMake logic that detects the problematic
case and forces the optimization level for those two files down to -O1 if necessary, but avoids
bumping it up in case of a debug build.

When building libdunealugrid as a shared library, linking fails (at
least on OS X) due to missing references to symbols in the core
libraries.

This patch fixes the problem by explicitly linking against the Dune
libraries.

Separate topology traits data and accessor functions

See merge request extensions/dune-alugrid!47

This avoids compiler warnings about declared but not defined static
members.

[test-hierarchic] Stop using deprecated GeometryType constructor.

See merge request extensions/dune-alugrid!44

Copy and adapt the Gmsh test from dune-grid.

Bugfix datahandle contains for specialization of codim 0.

Closes #41

See merge request extensions/dune-alugrid!40

GatherScatterBaseImpl< codim=0 >.

ALUGridCube.

-DALUGRID_DISABLE_METIS=FALSE is specified.

[bugfix] remove spurious include to rpc/rpc.h

See merge request extensions/dune-alugrid!38