Compiling libelas as static library

• petrack_core is no longer built using -fpermissive
• libelas is compiled as static library and then linked to petrack
• libelas is now turned OFF by default (since stereo functionality as a whole is)

When libelas *is *turned on, we still see the warnings (-fpermissive cannot be silenced; its already deprecation errors into warnings). One could first compile libelas and then the PeTrack target, to have less clutter.

Since fpermissive is mandatory, this does mean nonconformant code is used. We should evaluate libelas as a depndecy as a whole (is it updated regularly, on what compilers does it work, what alternatives are there) once we choose to reintroduce the stereo functionality.

Closes #12 (closed)