Compile libelas as static library

Libelas cannot be commpiled without the fpermissive flag. With PeTrack itself, we do not want to be that permissive.

As a solution, we could put libelas into a static library, which is compiled with fpermissive and link it to PeTrack. This way libelas can be compiled with fpermissive without the rest of PeTrack having the same flag.