posix: threads: do not blanket-define _POSIX_THREADS via cmake

[cfriedt]

Describe the bug

Prior to #51211, POSIX features were mostly all available all the time when CONFIG_POSIX_API was selected.

As part of the above ticket, POSIX features via Kconfig options that were crafted to resemble standard Options and Option Groups.

With that, we no longer need to blanket-define -D_POSIX_THREADS, which was originally required with external C libraries such as Newlib and Picolibc.

In fact, if _POSIX_THREADS is enabled, and CONFIG_POSIX_THREADS is not selected, it is a bug.

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? all
• What have you tried to diagnose or workaround this issue? a lot
• Is this a regression? no, it is a workaround that has existed in-tree for many years

To Reproduce

1. Select minimal Kconfig options in prj.conf
2. Select CONFIG_EVENTFD=y
3. Add a preprocessor statement of the form

#if defined(_POSIX_THREADS)
#ifndef CONFIG_POSIX_THREADS
#error Feature _POSIX_THREADS is enabled but CONFIG_POSIX_THREADS is not selected (inconsistence)
#endif
#endif

Expected behavior
POSIX features should match the configuration

Impact
It introduces an element of instability / unexpected behaviour.

Logs and console output

Feature _POSIX_THREADS is enabled but CONFIG_POSIX_THREADS is not selected (inconsistence)

Environment (please complete the following information):

• OS: any
• Toolchain (e.g Zephyr SDK, ...): 0.16.8
• Commit SHA or Version used: 4081098

Additional context

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.