Pass pandoc a more-compatible Markdown flavor
Pass pandoc a more-compatible Markdown flavor

Currently, the generated manpages in tarballs are different than the
ones that CI generates, as Pandoc's default markdown flavors are
inconsistent across versions. Previous commit in this series attempted
to bridge the difference, but failed to work in older versions of
Pandoc.

In Pandoc < v2 (like e.g. Ubuntu 18.04's v1.19, used in CI):
  * extension "smart" does not exist, there is a CLI option -S instead
  * "markdown" does not enable any "smart" features by default
  * "markdown_strict" and "markdown_mmd" do not include extension
     backtick_code_blocks (which we use)
  * GitHub-flavored Markdown is "markdown_github"

Pandoc v2+ (like e.g. v2.9 used by current upstream tarballs):
  * Added the smart extension (-smart/+smart), deprecating -S
  * The default Pandoc flavor ("markdown") enables "smart" by default
  * "markdown_github" is deprecated, "gfm" is used instead

Therefore, to achieve the desired result of a) not including "smart" b)
enabling backtick code blocks, our only option is to specifically use a
Markdown flavor that does not include smart in either version
("markdown_strict" or "markdown_mmd"), plus specifically enable the
"backtick_code_blocks" extension. Since we want to enable
"mmd_title_block" anyway (for lowdown compatibility), use
"markdown_mmd+backtick_code_blocks".

This has been confirmed to generate the same output as before these
changes on both Ubuntu 18.04 (pandoc 1.19) and Debian sid (pandoc 2.9).

maxmind/libmaxminddb,maxmind/libmaxminddb,maxmind/libmaxminddb