Try to fix parallel docs builds with subdirs

[jimklimov]

Picks up from #1152 to allow actual parallel builds (and distchecks) with each piece of documentation rolling in a dedicated uniquely named directory while it is being generated. This allows to avoid the conflict of requesting same-named targets from different rules, as well as of a2x using same-named helper files to build different docs, ruling it out on the OS/filesystem level. And to gain parallelism instead of safely evading it with earlier trickery attempts.

Subject also touched in #505 and #418 discussions.

[jimklimov]

TODO: If the different make implementations in CI farm (GNU, BSD, Sun) all do not choke on this change of recipe for at least sequential builds, next step is to activate parallel rules in Jenkinsfile-dynamatrix to gain the benefit on CI farm. Or maybe, temporarily, to add such scenarios as "permitted to fail", to see if this change causes any intermittent/racey non-100% faults.

[jimklimov]

For a rough benchmark, on a laptop this branch passed make clean -k; make distcheck -j15 in 2m4s (there are far less than 15 cores, so this was more to check for collisions), and a sequential make clean -k; make distcheck in 4m18s. Both succeeded. (C/C++ overhead was negligible thanks to ccache; so this was mostly configure, make and asciidoc/a2x CPU time).

Master branch passed seq build in 4m5s at the same timeframe.