Fix OS availability check of pthread_threadid_np for iOS (#2897)

include/spdlog/details/os-inl.h

@@ -332,8 +332,10 @@ SPDLOG_INLINE size_t _thread_id() SPDLOG_NOEXCEPT {
     return static_cast<size_t>(::thr_self());
 #elif __APPLE__
     uint64_t tid;
-    // There is no pthread_threadid_np prior to 10.6, and it is not supported on any PPC,
+    // There is no pthread_threadid_np prior to Mac OS X 10.6, and it is not supported on any PPC,
     // including 10.6.8 Rosetta. __POWERPC__ is Apple-specific define encompassing ppc and ppc64.
+    #ifdef MAC_OS_X_VERSION_MAX_ALLOWED
+    {
         #if (MAC_OS_X_VERSION_MAX_ALLOWED < 1060) || defined(__POWERPC__)
         tid = pthread_mach_thread_np(pthread_self());
         #elif MAC_OS_X_VERSION_MIN_REQUIRED < 1060
@@ -345,6 +347,10 @@ SPDLOG_INLINE size_t _thread_id() SPDLOG_NOEXCEPT {
         #else
         pthread_threadid_np(nullptr, &tid);
         #endif
+    }
+    #else
+    pthread_threadid_np(nullptr, &tid);
+    #endif
     return static_cast<size_t>(tid);
 #else  // Default to standard C++11 (other Unix)
     return static_cast<size_t>(std::hash<std::thread::id>()(std::this_thread::get_id()));