configs: enable libstdcxx-threads and libstdcxx-time via posix

[cfriedt]

Enable standard C++ threads and time functionality via Zephyr's POSIX API.

---

• GCC PR: libstdc++-v3: enable std::thread for zephyr via posix gcc#25

[cfriedt]

The added ./configure options might kick off some autotools tests (GLIBCXX_CHECK_GTHREADS, GLIBCXX_ENABLE_LIBSTDCXX_TIME, ..) which I am skeptical about. Those, in turn, run some other autoconf build-time tests.

Until we have a stable interface based on *zephyr*, and if this change does not work as-is, it might be necessary to hard-code the following in gcc/libstdc++-v3/acinclude.m4.

  ac_has_clock_monotonic=yes
  ac_has_clock_realtime=yes
  ac_has_nanosleep=yes
  ac_has_sched_yield=yes

Even hard-coding the above values might not work, in which case we will likely need to provide some modified means of configuration - e.g. manually using AC_DEFINE() and AC_SUBST() macros to enable the needed options.

Eventually, we definitely do want to support target-based feature selection, such that we can set sane defaults via target *zephyr* and not run any build-time tests.

[cfriedt]

@stephanosio, @keith-packard -

The changes here should be just about sufficient to enable std::thread and friends. Cannot rely on compile or link tests here obviously (unless there was some kind of fake set up for Zephyr x-toolchain builds, which might be an option).

I dropped a previous commit (see paste below) that polluted gthr-posix.h with a bunch of declarations.

https://pastebin.com/Sb1XUQ9H

Those declarations should be in <pthread.h> or <time.h> with just one or two feature test macros but none of them show up in CI for some reason.

Any suggestions?

https://github.com/zephyrproject-rtos/picolibc/blob/main/newlib/libc/machine/cris/include/pthread.h
https://github.com/zephyrproject-rtos/picolibc/blob/main/newlib/libc/include/time.h

https://github.com/zephyrproject-rtos/newlib-cygwin/blob/zephyr-newlib-3.3.0/newlib/libc/include/pthread.h
https://github.com/zephyrproject-rtos/newlib-cygwin/blob/zephyr-newlib-3.3.0/newlib/libc/include/time.h

Note: These feature test macros should only be defined for the build. We cannot hard code them into an installed header, since Zephyr apps might disable POSIX support.

[cfriedt]

It's too bad there are no actual instructions for building the SDK. E.g. the last time I had to change something in the SDK or otherwise build it from scratch, I needed to make a bash script for doing so.

I'm able to build the SDK manually without issue using unmodified changes in this PR and in zephyrproject-rtos/gcc#25 .

build.log.xz.txt (xz -d < build.log.xz.txt > build.log.txt)

Not 100% sure how to fix CI here, but it does not seem to be working.

[cfriedt]

Closing this in favour of using the ISO C11 threads API, as there are fewer moving parts and Zephyr does not yet have a "kernel headers" package to use in constructing the toolchain (or a numeric syscall system / ABI).