Fixes for building on centos7 and centos8

[quanah]

These changes enable me to build git-buildpackage on CentOS8 without any errors.

[agx]

Thanks! I Two nits above, if that's too much of a hazzle i can split it out when merging.

[quanah]

Done, let me know if there's anything further. :)

[agx]

@quanah I've merged the first bit but while testing i noticed that the new logic prefers docbook2man over docbook2x-man which breaks the doc build on Debian. Can you make the logic select docbook2x-man with highest preference (or at least let me know if that works for you too and doesn't break your Centos build right away again)

[quanah]

@quanah I've merged the first bit but while testing i noticed that the new logic prefers docbook2man over docbook2x-man which breaks the doc build on Debian. Can you make the logic select docbook2x-man with highest preference (or at least let me know if that works for you too and doesn't break your Centos build right away again)

Let me know if you like the new solution better. :)

[agx]

@quanah looks good, i've split things up a little (keeping your authorship) and pushed.