Fix build with latest asciidoc (#805)
Fix build with latest asciidoc (#805)

asciidoc in debian:sid is now a bash script to force it to run with
python3. This breaks our workarounds for older versions. This patch
extends the workarounds to support the new case.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl