The libslvs library and CDemo program can now be built by Autotools. A few
code changes were needed for this: C++ comments in C code had to be
converted, constraint.cpp required some massaging, and fltkutil.cpp needed
a stub for InitHeaps().
the documentation accordingly. Also rename the C example for
consistency, and update copyright dates.
[git-p4: depot-paths = "//depot/solvespace/": change = 2190]