Change regex used to detect GNU sed.

src/std/text.ab

@@ -17,7 +17,7 @@ pub fun replace_regex(source: Text, pattern: Text, replacement: Text, extended:
  // GNU sed versions 4.0 through 4.2 support extended regex syntax,
  // but only via the "-r" option; use that if the version information
  // contains "GNU sed".
- unsafe $re='\bGNU sed\b'; [[ \$(sed --version 2>/dev/null) =~ \$re ]]$
+ unsafe $re='\bCopyright\b.+\bFree Software Foundation\b'; [[ \$(sed --version 2>/dev/null) =~ \$re ]]$
  let flag = status == 0 then "-r" else "-E"
  return $echo "{source}" | sed {flag} -e "s/{pattern}/{replacement}/g"$
  } else {