testsuite: optional 'parallel' target uses GNU parallel to halve testing time

[gasche]

See the run logs below. Using 'parallel' reduces the time to run the
full testsuite from 2m30s to 0m57s on my machine.

All tests are run in parallel, including lib-threads.

We ask parallel to preserve output order, so it should be
deterministic. (Individual tests may mangle stdout and stderr in
fragile ways, though.) See the Makefile comment for details.

rm -f _log; make clean;
time make all;
cp _log /tmp/trunk_log

> real  2m29.947s
> user  1m35.937s
> sys   0m17.467s

rm -f _log; make clean;
time make parallel;
cp _log /tmp/parallel_log

> real  0m56.611s
> user  2m20.861s
> sys   0m21.511s

diff -u /tmp/{trunk,parallel}_log | wc -l

> 0

rm /tmp/{trunk,parallel}_log

[gasche]

cc @damiendoligez

[mshinwell]

This sounds lovely.

[damiendoligez]

You should also update the help text in the default target, and maybe add a Changes entry?

[gasche]

@damiendoligez I included a Changes entry (ah, those pesky maintainer requirements...)

I also included some logic to fail informatively when the tool is not present on the user's machine. The current output if you run a parallel* target on a machine that does not have parallel is the following:

/bin/sh: parallel: command not found
Unable to run the GNU parallel tool;
You should install it before using the parallel* targets.
Makefile:79: recipe for target 'parallel-typing' failed
make: *** [parallel-typing] Error 1

For the record, there also exists a moreutils package that provides a parallel tool that is similar but has incompatible command-line (and doesn't seem to have the --keep-order option). GNU parallel seems to be the default in Fedora and Debian nowadays. I don't think it is possible to write the script to work with both tools; if you installed moreutils-parallel, running a parallel* target will fail with the following output:

Your 'parallel' tool seems incompatible with GNU parallel;
This target requires GNU parallel.
Makefile:79: recipe for target 'parallel-typing' failed
make: *** [parallel-typing] Error 1