Fix handling of ELPA with openmp

The new ELPA finder module uses the input variable ELPA_PREFER_OPENMP to search for the right flavor of ELPA. Due to an oversight, this variable was not set properly, resulting in OpenMP-enabled versions of ELPA not being found by CMake.

Now this variable is actually a dependent option, which is hidden (and OFF) if SIESTA_WITH_OPENMP is OFF, and defaults to ON if SIESTA_WITH_OPENMP is ON (but can be set to OFF if desired).

Fixes #574 (closed)