Fail gracefully with ancient /usr/bin/man version

man version 1.x does not support local file mode.  sfdoc is mostly for
development purposes, so instead of jumping through hoops to make it
work; we assume user can view the pages in other format or on a less
obsolete distro.

src/bin/sfdoc

         man)
             which pod2man &>/dev/null \
              || die "pod2man is missing cannot use man page mode"
+            man --version |& grep -F 'man 2.' \
+             || die "unknown \`man\` (only 2.x is supported): $(man --version 2>&1)"
             sfdoc__export manpage "${RealModuleName:-$module}" "$mpath" "$encoding" \
               | man -l -
             ;;