ELPA tends to pull in OpenMP

Description

Currently ELPA is being searched with find_package(ELPA MODULE OPTIONAL_COMPONENTS OpenMP) which uses pkg-config to find elpa_openmp.pc or, if that is not found, elpa.pc. However, in elpa_openmp.pc there is a hardcoded -fopenmp which then propagates into the compiler and linker flags of the Octopus targets. To prevent that we only add COMPONENTS OpenMP when OCTOPUS_OpenMP is enabled.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.