syntaxes: add author and license lines to four of the files

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>

syntax/autoconf.nanorc

@@ -1,5 +1,8 @@
 ## Syntax highlighting for Autoconf.
 
+## Original author:  Benno Schulenberg
+## License:  GPL version 3 or newer
+
 syntax autoconf "\.(ac|m4)$"
 comment "#"
 

syntax/changelog.nanorc

@@ -1,5 +1,8 @@
 ## Syntax highlighting for Changelogs.
 
+## Original author:  Benno Schulenberg
+## License:  GPL version 3 or newer
+
 syntax changelog "Change[Ll]og[^/]*$"
 
 # Author lines.

syntax/po.nanorc

@@ -1,5 +1,8 @@
 ## Syntax highlighting for PO files.
 
+## Original author:  Benno Schulenberg
+## License:  GPL version 3 or newer
+
 syntax po "\.pot?$"
 comment "#"
 

syntax/texinfo.nanorc

@@ -1,5 +1,8 @@
 ## Syntax highlighting for Texinfo files.
 
+## Original author:  Benno Schulenberg
+## License:  GPL version 3 or newer
+
 syntax texinfo "\.texi$"
 header "^\\input texinfo"
 magic "Texinfo source"