The current Bison release 2.6 removed support for long-deprecated YYPARSE_PARAM, which XPathGrammar.y in the webnode tree uses. Instead, reentrant Bison grammars should use %parse-param, which has been supported in Bison going back to 2.3 (2006). This change has been made in upstream WebKit. See https://bugs.webkit.org/show_bug.cgi?id=92264

I think it would be bad to open-source the WebView component in a state where it can't be built using current tools. That adds an extra documentation and maintenance hassle for us and it annoys developers. I'm told a merge from upstream WebKit won't happen until after the open-sourcing.

I suggest a patch based on upstream's patch, such that a future merge from upstream won't be made more complicated.
I have such a patch - see the attachment use-new-bison.patch.