Meson: Fallback for older Pandoc versions

Pandoc >=2.0.0 needs "-f markdown-smart", which requests (amongst some
other things) not to convert "--" to an en-dash.  We have a lot of
double hyphens in the documentation, and I would prefer not to have to
escape every single one of them.

Older Pandoc versions don't recognise "-smart", but also don't do the
en-dash thing by default.  Here, we can simply use "-f markdown".

Note that older Pandoc versions don't quite render the manual pages
correctly, but it's near enough.

Fixes: https://gitlab.desy.de/thomas.white/crystfel/-/issues/90

0 files changed, 0 insertions, 0 deletions