Update o3dgcTimer.h

closes https://github.com/assimp/assimp/issues/1642: fix build on Hurd

contrib/Open3DGC/o3dgcTimer.h

@@ -30,7 +30,7 @@ THE SOFTWARE.
 /* Thank you, Microsoft, for file WinDef.h with min/max redefinition. */
 #define NOMINMAX
 #include <windows.h>
-#elif __MACH__
+#elif __APPLE__
 #include <mach/clock.h>
 #include <mach/mach.h>
 #else
@@ -73,7 +73,7 @@ namespace o3dgc
         LARGE_INTEGER m_freq;
 
     };
-#elif __MACH__
+#elif __APPLE__
     class Timer
     {
     public: