Suitesparse, as installed with Debian, is thread-parallel using OpenMP.
OpenMP silently assumes, it can spawn as many threads as there are cores.
In a worst case scenario, this leads to a number of threads quadratic in
the core count, if you also do parallel test execution with the maximum
number of cores. We solve the issue by disallowing OpenMP to allocate more
than one thread.

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

- No longer test with clang 3.5.
- Add clang 3.8 from Debian 8 + backports
- Add gcc 5.4 + clang 3.8 from Ubuntu 16.04 LTS.
- Use `duneci-standard-test`