Stop copying the licence text into C source code.

Stop copying the licence text into C source code.

Now all the uses of the licence text or the short copyright notice get
it from a new header "licence.h", which in turn is built by a Perl
script licence.pl invoked by mkfiles.pl, using LICENCE itself as the
source.

Hence, I can completely remove a whole section from the list of
licence locations in CHECKLST.txt :-)