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>