Other implementations should only be selected if the compiler is specifically
told to support them.

Closes: #165

All of the other modules don't specify the quick label on their tests.

The special vectorization test run stays in the local configuration, as it is specific to
dune-common.

This adds build that runs expensive tests, including Vc on with at least AVX.
The build is allowed to fail to compensate for the possibility of no runner
beeing available.

Debian 8 is not supported anymore.

This way dependencies installed via `duneci-install-module` are built
with the same options as the module to be tested.

 - Added:   Debian 10 with gcc in C++17 mode
 - Removed: Debian 8 + Backports with clang

Reference: http://lists.dune-project.org/pipermail/dune-devel/2017-July/002224.html

gcc 5.4 and clang 3.8.

Reference: core/dune-common#37