threads: do not use libdispatch where it is not present

Fixes: #850

Author: barracuda156

common/threads.cpp

@@ -128,7 +128,8 @@ void althrd_setname(const char *name [[maybe_unused]])
 #endif
 }
 
-#ifdef __APPLE__
+/* Do not try using libdispatch on systems where it is absent. */
+#if defined(__APPLE__) && ((MAC_OS_X_VERSION_MIN_REQUIRED > 1050) && !defined(__ppc__))
 
 namespace al {
 

common/threads.h

@@ -15,7 +15,12 @@
 #endif
 
 #if defined(__APPLE__)
+#include <AvailabilityMacros.h>
+#if (MAC_OS_X_VERSION_MIN_REQUIRED > 1050) && !defined(__ppc__)
 #include <dispatch/dispatch.h>
+#else
+#include <semaphore.h> /* Fallback option for Apple without a working libdispatch */
+#endif
 #elif !defined(_WIN32)
 #include <semaphore.h>
 #endif
@@ -27,7 +32,7 @@ namespace al {
 class semaphore {
 #ifdef _WIN32
     using native_type = void*;
-#elif defined(__APPLE__)
+#elif defined(__APPLE__) && ((MAC_OS_X_VERSION_MIN_REQUIRED > 1050) && !defined(__ppc__))
     using native_type = dispatch_semaphore_t;
 #else
     using native_type = sem_t;