ipc*: Support build in C++20 mode; #include of our headers when compiling in C++20 mode.

[ygoldfeld]

This is the Flow-IPC equivalent of Flow-IPC/flow#95, as applied to not just Flow but the other parts of Flow-IPC. So basically Flow-IPC "proper." Please read that ticket and apply those words to us here.

Differences/additions:

• After the aforementioned (Flow-specific) issue is handled, we can reuse much of that work (since FlowLike*.cmake is used by Flow-IPC build script(s)); and use the same techniques/lessons otherwise.
• So far the business case for C++20 support for Flow-IPC has not come up. (It has for Flow.) I anticipate that won't last though. So might as well handle it in short order after the above ticket.
• There might be an added complication. A colleague has reported that building Cap'n Proto (capnp) in C++20 mode at least with a certain somewhat older gcc version causes an internal compiler error to pop up, halting the capnp build. capnp is a key dependency of ipc_transport_structured and its dependents ipc_session, ipc_shm, ipc_shm_arena_lend. Basically - we'll just have to see what the CI pipeline reveals on that matter; then handle it best we can.