[CMake,ML] Fixed OpenMP compiler flag.
Fixes #3295 (closed)
Please note that you need to set the new option -DOGS_USE_MKL_EIGEN_OPENMP=ON
-DOGS_USE_PETSC_MKL_EIGEN_OPENMP=ON
to enable OpenMP in Eigen when using OGS_USE_MKL=ON
and OGS_USE_PETSC=ON
.
Edited by Lars Bilke