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.

misc/update_config_scripts

@@ -3,7 +3,7 @@
 # Update the bundled scripts config.guess and config.sub
 # for the Fast Light Tool Kit (FLTK).
 #
-# Copyright 1998-2017 by Bill Spitzak and others.
+# Copyright 1998-2021 by Bill Spitzak and others.
 #
 # This library is free software. Distribution and use rights are outlined in
 # the file "COPYING" which should have been included with this file.  If this
@@ -24,19 +24,18 @@
 # browser to view and download the files if wget is not installed on
 # your system.
 #
-# After updating the scripts, copy config.guess and config.sub
-# to the FLTK root directory, test, and eventually commit the changes:
+# After downloading the scripts, copy config.guess and config.sub to
+# the FLTK root directory, test, and eventually commit the changes:
 #
 # # cd to FLTK root directory
 #
 # $ make distclean
-# $ rm -f config.guess config.sub configure
-# $ cp misc/config.guess misc/config.sub .
 # $ autoconf -f
 # $ ./configure [options ...]
 # $ make
 # # test
-# $ svn commit
+# $ git add config.guess config.sub
+# $ git commit
 #
 
 for FILE in config.guess config.sub; do