libpq-oauth uses floor() but did not link against libm. Since libpq
itself uses -lm, nothing in the buildfarm has had problems with
libpq-oauth yet, and it seems difficult to hit a failure in practice.

But commit 1443b6c attempted to add an executable based on
libpq-oauth, which ran into link-time failures with Clang due to this
omission. It seems prudent to fix this for both the module and the
executable simultaneously so that no one trips over it in the future.

This is a Makefile-only change. The Meson side already pulls in libm,
through the os_deps dependency.

Backpatch-through: 18

To better record the internal behaviors of oauth-curl.c, add a unit test
suite for the socket and timer handling code. This is all based on TAP
and driven by our existing Test::More infrastructure.

This commit is a replay of 1443b6c, which was reverted due to
buildfarm failures. Compared with that, this version protects the build
targets in the Makefile with a with_libcurl conditional, and it tweaks
the code style in 001_oauth.pl.

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/CAOYmi+nDZxJHaWj9_jRSyf8uMToCADAmOfJEggsKW-kY7aUwHA@mail.gmail.com

Requires Python 3. On the first run of `make installcheck` or `meson
test` the dependencies will be installed into a local virtualenv for
you. See the README for more details.

Cirrus has been updated to build OAuth support on Debian and FreeBSD.

The suite contains a --temp-instance option, analogous to pg_regress's
option of the same name, which allows an ephemeral server to be spun up
during a test run.

TODOs:
- The --tap-stream option to pytest-tap is slightly broken during test
  failures (it suppresses error information), which impedes debugging.
- The with_oauth test skip logic should probably be integrated into the
  Makefile side as well...
- See if 32-bit tests can be enabled with a 32-bit Python.