Resolve "[23b] kxc variant for libxc"
Add kxc variant for libxc.
Closes #37 (closed) https://github.com/mpsd-computational-science/spack/pull/9 and https://github.com/spack/spack/pull/38937/files
Add kxc variant for libxc.
Closes #37 (closed) https://github.com/mpsd-computational-science/spack/pull/9 and https://github.com/spack/spack/pull/38937/files