The script config.pl fails when setting a #define symbol if the symbol isn't
already in the configuration header. This adds an option '--force' to append
the symbol to the end of the file if it isn't already present.
Also clarified usage, and added copyright to the config.pl.
Otherwise we get warnings that some documentation items don't have
corresponding #define, and more importantly the corresponding snippets are not
included in the output.
For that we need a modified version of the "full" argument for config.pl.
Also, the new CMakeLists.txt target only works on Unix (which was already the
case of the Makefile target). Hopefully this is not an issue as people are
unlikely to need that target on Windows.
