Running autoconf gave me the following error messages:
configure.ac:129: warning: AC_OUTPUT should be used without arguments.
configure.ac:129: You should run autoupdate.
It seems that the syntax used on line 129 has been deprecated (possibly in the previous century :-[ )
Running autoupdate corrected this and what I assume is another minor syntax error.
Standard's version of configure.ac seems to have already made these changes.
Pull request #544 (https://github.com/jamespetts/simutrans-extended/pull/544) incorporates autoupdates's suggestions.
So I have done some basic due diligence but am fairly out of my depth here. :exclaim:
Now incorporated - thank you.