build: keep CC free of -std=gnu23 for the ucx-spcx-plugin build (#2029)

## Summary

Ports #2013 from `release/1.4.0` to `main` — main carries the same
latent failure.

Since #1991 pinned `gcc-toolset-14` ahead of the system gcc on `PATH`,
the plugin's `AC_PROG_CC` detects C23 support and resolves `CC` to `gcc
-std=gnu23`. The plugin's `src/gdp/Makefile.am` forwards the compiler
unsplit as `-hostcc=$(CC)`, so dpacc receives a bare `-std=gnu23`
argument and dies with `Unknown option` / exit 255, failing every
python/arch cell of a spcx-enabled wheel build.

- Preset `ac_cv_prog_cc_c23=no` before the plugin's `configure`, so
autoconf falls back to C11 — already covered by gcc 14's default gnu17 —
and `CC` stays plain `gcc`.

`--build-ucx-spcx-plugin` is not enabled in the PR CI pipeline (it is
consumed by the QA/release invocations), so this does not reproduce on
per-PR builds — it surfaces on the next QA/release build cut from main.
Verified fixed on the 1.4.0 release build.

This is a workaround on the nixl side. The underlying bug is in the
plugin, which should split the compiler from its flags
(`-hostcc=$(firstword $(CC))`, remainder into `--hostcc-options`); the
comment notes the revert condition.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Bug Fixes**
* Fixed optional UCX SPcX plugin builds in manylinux environments by
preventing unsupported C23 compiler options from being passed during
compilation.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: NirWolfer <nwolfer@nvidia.com>