Fix README files and remove stale config.guess and config.sub

README.Unix.txt and README.Windows.txt (MinGW, MSYS) instructed to
execute `NOCONFIGURE=1 ./autogen.sh` which is no longer necessary
(a simple `autoconf` does the job).

Remove files misc/config.guess and misc/config.sub because they are
no longer needed: these files are now in th FLTK root directory
available. Remove all references to these files.

Fix .gitattributes accordingly. Also exclude the .github folder
from exports, for instance when creating distribution tarballs.