Add MinGW build support for the C client

[yeads]

Summary

• distinguish MSVC and MinGW compiler flags in the CMake build
• make Winsock and IPv6 feature detection work with MinGW-w64, including 32-bit stdcall symbols
• preserve GCC attributes and POSIX-compatible declarations when compiling with MinGW
• fix Windows socket handling, timeout configuration, monotonic clock selection, and CLI socket types
• fix Windows thread-local storage and thread handle management in the pthread compatibility layer
• document 32-bit and 64-bit MinGW-w64 cross-build commands

Testing

• built the synchronous client library and CLI with x86_64-w64-mingw32-gcc
• built the synchronous client library and CLI with i686-w64-mingw32-gcc
• built the asynchronous client library and CLI with x86_64-w64-mingw32-gcc
• built with OpenSSL 4.0.1 from /usr/local/mingw in both shared-library and static-library modes
• verified that the shared build imports libssl-4-x64.dll and that the static build has no OpenSSL DLL dependency
• repeated MinGW builds with implicit declarations, incompatible pointer types, integer conversions, and missing returns promoted to errors
• completed a native Linux CMake build as a regression check
• verified the generated executables as PE32 and PE32+ artifacts

Cyrus SASL was disabled for the cross-build checks because a matching MinGW build was not available in the test environment. Existing defaults and optional feature support remain unchanged.