- naive circular queue, described in the article "A Scalable, Portable, and
 Memory-Efficient Lock-Free FIFO Queue" by Ruslan Nikolaev:
https://drops.dagstuhl.de/opus/volltexte/2019/11335/pdf/LIPIcs-DISC-2019-28.pdf
- circular queue, proposed by Valentin Bartenev in the "Unit router application
IPC" design draft

The gem paths must depend on the specified interpreter.
Also, gemdir looks redundant as it's already included in Gem.default_path().

There is no reason to use printf instead of just print.

No functional changes.

An attempt to build a Ruby module for a custom Ruby installation that has the
same major version as the system Ruby may unexpectedly cause the use of the
system Ruby library.

This closes #449 issue on GitHub.

Since the introduction of rootfs feature, some language modules
can't be configured multiple times.

Now the configure generates a separate nxt_<module>_mounts.h for
each module compiled.

Some PPAs for Ubuntu package PHP with versions like:
 7.2.28-3+ubuntu18.04.1+deb.sury.org+1

But the script expected only "X.Y.Z".

The issue was introduced in:
 http://hg.nginx.org/unit/rev/2ecb15904ba5