cmake: use OpenMP imported target / cleanup
cmake: use OpenMP imported target / cleanup

- flags: O3 include ftree-vectorize
- remove commented statements

electronic-structure/sirius,electronic-structure/sirius,electronic-structure/sirius,electronic-structure/sirius,toxa81/sirius,toxa81/sirius,electronic-structure/sirius,toxa81/sirius,toxa81/sirius,electronic-structure/sirius,toxa81/sirius,toxa81/sirius,electronic-structure/sirius