Examples were relicensed (under permission from JJGR) to BSD-2-Clause a few
years back. We also remove Copyright from individual files to match the
convention that examples doesn't have that (whole codebase is under JJGR's
copyright too).
Do not include `gc.h' in the example (only ecl/ecl.h). Things work as
expected whatsoever and we don't put gc.h anywhere for quite a
while. Fixes#136.
Also replace obsolete Cnil with ECL_NIL.