This required some adjustments:
* New warnings character-conversion and deprecated-volatile are
disabled for the reasons indicated in the adjacent comments.
* C23 is a lot stricter about function pointer signatures so some
callbacks in gnc-frequency.c and dialog-sx-editor had to be cast using
the long-available GLib macros.
* C++23 is stricter about a reference (foo&) parameter taking an
lvalue only; a lambda used in several csv-importer files was trying to
pass an rvalue so the parameter had to be changed to perfect
forward (foo&&).
* C++23 doesn't allow oring together enum values from different enum
types, even anonymous ones, so the two gnc_numeric enums for rounding
and denominators are combined into a single enum GncNumericRoundDenom.
* C++20's new automatic generation of comparison functions combined
with its stricter automatic conversion rules caused trouble with
operator==() and operator!=() for gnc::GUID and GncGUID, complaining
about ambiguity or not being able to find operator==(const GncGuid&,
const GncGuid&). The fix has two parts: Changing operator!=(const
gnc::GUID&, const gnc::GUID&) to operator==(const gnc::GUID&, const
gnc::GUID&) to resolve the conversion problem and changing the call
forwarding operator==(const GncGuid&, const GncGuid&) to only cast the
first argument to gnc::GUID so that it could work unambiguously with
operator==(const gnc::GUID&, const GncGuid&).
* python sqlite3test: C23 apparently makes AppleClang a lot pickier
about includes.
ci-tests.yml runs Ubuntu 18.04 and Ubuntu 20.04 using Github-provided virtual environments. The virtual environments are about twice as fast as Docker ones and getting Docker to run Ubuntu images was problematic. Note that updating Ubuntu 18.04 updates ICU4C from v60 to v65 and that causes conflicts with Boost, so don't.
ci-docker.yml runs tests in a Docker container running Arch Linux.
mac-tests.yml runs tests on macOS using a Github-provided virtual machine. Note that this test relies on a prebuilt tarball containing all of the dependencies. Instructions for building that tarball along with some support scripts may be found in utils/ci/macos-ci-deps.