actually I don’t use a separate BLAS. I’m not sure what happens internally. I guess Eigen has its own BLAS.
Anything Jörg recommend so far is correct, the missing puzzle piece in my case was to remove the old MKL
sudo apt remove intel-mkl
and possibly autoremove, so that OGS solely relies on the latest MKL from Intel.
Another useful hint by Aqeel was to use the clang compiler, alternatively, thanks to Jörg, you may select another linker https://gitlab.opengeosys.org/ogs/ogs/-/commit/02967311512e5b9e2e747cd2b6832e17a54140f3
It seems to be that this commit is really breaking things with some configurations.
I opened an issue: 02967311512e5b9e2e747cd2b6832e17a54140f3 causes undefinded symbol error on some machines when compiled intel MKL (#3259) · Issues · ogs / ogs · GitLab
There, I also propose another fix.