ci: Explicitly enable Meson features

ci: Explicitly enable Meson features

Meson's "auto" feature mode silently disables features with missing
prerequisites, which is nice for development but can lead to false
positives in the CI (such as my commit b0635bfda, which broke OAuth
detection on OpenBSD). Use an explicit feature list in the Cirrus config
instead; this mirrors the --with-XXX experience of Autoconf.

While we're here, also move common configuration options into a single
variable, MESON_COMMON_PG_CONFIG_ARGS, as suggested by Peter. The
resulting hierarchy is as follows:

MESON_COMMON_PG_CONFIG_ARGS   "global" Meson configuration options

MESON_COMMON_FEATURES         the default set of CI features, to be used
                              unless there's a specific reason not to

MESON_FEATURES                per-OS feature configuration, overriding
                              the above

The current exceptions to the use of MESON_COMMON_FEATURES are
- SanityCheck, which uses almost no dependencies;
- Windows - VS, whose feature list has diverged significantly from the
  others; and
- Linux, which continues to use 'auto' features so that autodetection is
  still tested in the CI. (Options shared between 64- and 32-bit builds
  can go into LINUX_MESON_FEATURES instead.)

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Suggested-by: Jacob Champion <jacob.champion@enterprisedb.com>
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/flat/CAN55FZ0aO8d_jkyRijcGP8qO%3DXH09qG%3Dpw0ZZDvB4LMzuXYU1w%40mail.gmail.com