`__fallthrough` macro is not defined when building with Clang → implicit‑fallthrough warnings

[undici77]

Summary

The header include/utils/utils.h provides a portable shortcut for the GCC [[fallthrough]] attribute:

/* gcc attribute shorthands */
#ifndef __fallthrough
#if __GNUC__ >= 7
#define __fallthrough        __attribute__((fallthrough))
#else
#define __fallthrough
#endif
#endif

The current test only checks for GCC ≥ 7. When the project is compiled with Clang, __fallthrough expands to an empty token, so any use of __fallthrough; does not suppress the -Wimplicit-fallthrough warning (or the newer -Wswitch).

This results in noisy warnings on macOS builds that use Clang, even though Clang has supported __attribute__((fallthrough)) for many releases.

Environment

Item	Value
OS	macOS 26.0.1
Compiler	Apple clang version 17.0.0
Build system	CMake (default configuration)
Library version	master Commit c6d562e

Steps to reproduce

# 1️⃣ Clone the repository
git clone https://github.com/goToMain/libosdp.git
cd libosdp

# 2️⃣ Create a build directory and configure with clang
mkdir build && cd build
cmake ..

# 3️⃣ Build
make   # or `cmake --build .`

The macro expands to nothing, so the attribute is not applied.

Proposed fix

Extend the condition to also enable the macro when compiling with Clang:

/* gcc/clang attribute shorthands */
#ifndef __fallthrough
#if __GNUC__ >= 7 || defined(__clang__)
#define __fallthrough        __attribute__((fallthrough))
#else
#define __fallthrough
#endif
#endif

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