Use __has_feature(cxx_exceptions) to detect C++ exceptions on Clang

[vsaraikin]

Problem

MOODYCAMEL_EXCEPTIONS_ENABLED is auto-detected from __EXCEPTIONS on Clang/GCC. On Clang, __EXCEPTIONS means "some exception handling is available" — it is set for Objective-C exceptions even when C++ exceptions are disabled (-fno-exceptions). Compiling an Objective-C++ (.mm) translation unit that includes concurrentqueue.h with -fno-exceptions therefore enables the C++ throw/try paths and fails to compile.

This is the case reported in #435.

Fix

On compilers that provide __has_feature (Clang / AppleClang), use __has_feature(cxx_exceptions) — the authoritative query for whether C++ exceptions are enabled — instead of __EXCEPTIONS. The existing MSVC/GCC predicate is kept as the fallback for compilers without __has_feature.

#if defined(__has_feature)
#if __has_feature(cxx_exceptions)
#define MOODYCAMEL_EXCEPTIONS_ENABLED
#endif
#elif (defined(_MSC_VER) && defined(_CPPUNWIND)) || (defined(__GNUC__) && defined(__EXCEPTIONS)) || (!defined(_MSC_VER) && !defined(__GNUC__))
#define MOODYCAMEL_EXCEPTIONS_ENABLED
#endif

Behavior is unchanged on MSVC and GCC. On Clang, detection now tracks C++ exceptions specifically, so Objective-C++ with -fno-exceptions compiles correctly, and normal C++ builds with/without -fno-exceptions are detected as before. The user-overridable MOODYCAMEL_EXCEPTIONS_ENABLED escape hatch is untouched.

Refs #435.