[!227] Do not allow OpenMP to get more than one core in CI

Merge branch 'feature/disallow-multithreaded-suitesparse' into 'master'

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.

See merge request [!227]

  [!227]: gitlab.dune-project.org/core/dune-istl/merge_requests/227

.gitlab-ci.yml

@@ -2,6 +2,15 @@
 before_script:
   - duneci-install-module https://gitlab.dune-project.org/core/dune-common.git
 
+variables:
+# 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.
+  OMP_NUM_THREADS: 1
+
 debian:10  gcc:c++17:
   image: duneci/base:10
   script: duneci-standard-test