Merge branch 'thoennes/intel-override-flag' into 'master'

Thoennes/intel override flag

See merge request walberla/walberla!512

.gitlab-ci.yml

@@ -34,6 +34,7 @@ stages:
       - cd $CI_PROJECT_DIR/build
       - if dpkg --compare-versions `ompi_info | head -2 | tail -1 | sed 's/[^0-9.]*\([0-9.]*\).*/\1/'` ge 1.10; then export MPIEXEC_PREFLAGS="--allow-run-as-root" ; fi
       - cmake ..
+        -DCMAKE_CXX_FLAGS=$CMAKE_CXX_FLAGS
         -DWALBERLA_BUFFER_DEBUG=$WALBERLA_BUFFER_DEBUG
         -DWALBERLA_BUILD_TESTS=ON
         -DWALBERLA_BUILD_BENCHMARKS=ON