Quoting Amitai Schleier, who made this same change in ikiwiki[1],
where lots of the public-inbox highlight code comes from:
> As of 3.51, searchFile() is no longer provided in highlight's Perl
> bindings (at least on NetBSD and OS X, as built from pkgsrc). This
> leaves us falling through to getConfDir(), which has been gone
> rather longer.
>
> From highlight git, it appears searchFile() and getFiletypesConfPath()
> both originated in the 3.14 release. The latter is still available in
> 3.51, and returns the same result searchFile() used to. Switch to it.
So, this should still be compatible with the version of highlight.pm in
Debian, but add support for newer versions as well.
[1]: commit
4d06df9583e6c4145f8c6fc2fd51d7894c0b85ce
Cc: Amitai Schleier <schmonz-web-ikiwiki@schmonz.com>