Distributions uses generate-man.sh to create man pages, however it only assumes a container environment and attempts to build binaries towards `/go` which might be an illegal path in `fakeroot`. This patch ensures we only build `md2man` from the vendored sources if the command is not present. This allows distributions to supply thier packaged `md2man`. Signed-off-by: Morten Linderud <morten@linderud.pw>