Linux: Fix compiler warnings about function pointer comparisons and make linux build warning-clean

[ScottMunroMS]

on linux, the CURL_CALL abstraction causes warnings when used in runtime comparisons because the compiler correctly recognizes that it's (1) a dangerous pointer->bool comparison and (2) entirely pointless.

remove the extra runtime CURL_CALL checking on platforms which don't need it and add static_asserts to verify this is safe to do.

bonus, enable the linux build to make warnings errors with minor exceptions for external code.

warning-free code is safer code!

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR aims to make the Linux build warning-clean by removing pointless runtime CURL_CALL(...) availability checks (that trigger function-pointer comparison warnings) and replacing them with compile-time checks, while also tightening compiler warning settings.

Changes:

• Removed runtime CURL_CALL(curl_multi_poll) availability checks on non-GDK platforms and called curl_multi_poll directly when libcurl headers are >= 7.69.0.
• Added static_assert checks intended to validate unconditional availability of the required libcurl entry points.
• Enabled -Werror for the Linux target with a narrow warning suppression for external zlib sources, plus an additional warning suppression for the new static_assert pattern.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
Source/HTTP/Curl/CurlMulti.cpp	Replaces runtime function-pointer checks with compile-time assertions and direct curl_multi_poll/curl_multi_wait calls.
Build/libHttpClient.Linux/CMakeLists.txt	Enables warnings-as-errors for Linux builds and adds warning suppressions (zlib + tautological compare).