build: assume POSIX select() is available

[vszakats]

This change effectively replaces an explicit compile-time #error with
a missing prototype error in environments not offering select(), and
saves curl-compatible systems from performing an explicit feature check.

Refs:
https://pubs.opengroup.org/onlinepubs/009695399/functions/pselect.html
https://linux.die.net/man/2/select

[Copilot]

Pull request overview

This PR simplifies curl’s build-time feature detection by removing explicit select() availability checks/definitions and relying on existing platform assumptions (or compiler diagnostics) when poll() is unavailable.

Changes:

• Remove hard compile-time #error guards requiring select() (or poll()) in lib/select.c and lib/curlx/wait.c.
• Stop defining/detecting HAVE_SELECT across Autotools and CMake (including platform config headers and CMake caches).
• Remove the CURL_CHECK_FUNC_SELECT Autotools macro and its invocation.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/select.c	Removes the explicit “select or poll required” compile-time error and clarifies the select-based path when poll() is absent.
lib/curlx/wait.c	Removes the explicit compile-time error requiring select().
lib/curl_config-cmake.h.in	Drops HAVE_SELECT from the CMake-generated config header template.
lib/config-win32.h	Drops HAVE_SELECT from the Windows static config header.
lib/config-riscos.h	Drops HAVE_SELECT from the RISC OS static config header.
lib/config-os400.h	Drops HAVE_SELECT from the OS/400 static config header.
lib/config-mac.h	Drops HAVE_SELECT from the mac static config header.
configure.ac	Removes the CURL_CHECK_FUNC_SELECT invocation from Autotools configuration.
CMakeLists.txt	Removes the select symbol check that previously set HAVE_SELECT.
CMake/win32-cache.cmake	Removes cached HAVE_SELECT value.
CMake/unix-cache.cmake	Removes cached HAVE_SELECT value.
acinclude.m4	Removes the CURL_CHECK_FUNC_SELECT macro implementation.

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