build: assume snprintf() in mprintf, drop feature check

[vszakats]

• it was already required for curl_*printf() float/double support.
• some curl tests always fail without it.
• it was already assumed to be present to build test servers.
  Source code did not check for HAVE_SNPRINTF detection variable.
• it was already required to build examples.

Windows builds stopped using this detection and the function via earlier
commits.

Follow-up to 64f28b8 #20765
Follow-up to 935b1bd #9570 #9569

---

• rebase on build: drop unused snprintf() feature check on Windows #20790.

[vszakats]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: Removes build-time detection/defines for snprintf() and treats it as a required libc feature across both autotools and CMake.

Change: Simplifies lib/mprintf.c float/double formatting to always call snprintf() (or curlx_win32_snprintf on Windows), avoiding broken builds/tests when it’s absent.

_{🤖 Was this summary useful? React with 👍 or 👎}

[augmentcode]

Review completed. No suggestions at this time.

Comment augment review to trigger a new review at any time.

[Copilot]

Pull request overview

This PR simplifies libcurl’s build configuration by assuming snprintf() is available (needed for curl_*printf() float/double formatting) and removing the corresponding feature-detection plumbing.

Changes:

• Make lib/mprintf.c always use snprintf() for float/double formatting on non-Windows platforms.
• Remove HAVE_SNPRINTF generation from the CMake config header template and Unix cache.
• Drop snprintf() feature checks from both Autotools (configure.ac) and CMake (CMakeLists.txt).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/mprintf.c	Removes the non-snprintf() fallback path for float/double formatting (non-Windows now always uses snprintf()).
lib/curl_config-cmake.h.in	Removes the HAVE_SNPRINTF config header define template entry.
configure.ac	Stops checking for snprintf() in non-Windows function checks.
CMakeLists.txt	Stops checking for snprintf() in non-Windows function checks.
CMake/unix-cache.cmake	Removes cached HAVE_SNPRINTF value.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.