Minor autotools improvements.

Minor autotools improvements.

 - Don't use "foreign" option for automake. Without this automake will
   perform some additional sanity checks e.g. on missing files. It will also
   now add the usual INSTALL file which documents the configure options etc.

 - Add AUTHORS file.

 - Add 'check-news' automake option, which will abort 'make dist' if
   the NEWS file doesn't list the current (to be released) package
   version at the top (i.e., if we forget to update NEWS).