m4 licensing

[Eric Blake]

Based on the recent change to standards.texi:

| @@ -765,7 +765,8 @@ all-permissive license like this one:
|  @smallexample
|  Copying and distribution of this file, with or without modification,
|  are permitted in any medium without royalty provided the copyright
| -notice and this notice are preserved.
| +notice and this notice are preserved.  This file is offered as-is,
| +without warranty of any kind.
|  @end smallexample

Should we go ahead and modify all of gnulib's m4/*.m4 scripts to add that 
sentence?

-- 
Eric Blake