https://github.com/assimp/assimp/issues/728: XCode fix.

cmake-modules/AddGTest.cmake

@@ -10,7 +10,10 @@ endif()
 if (MSVC)
   set(RELEASE_LIB_DIR ReleaseLibs)
   set(DEBUG_LIB_DIR DebugLibs)
-else()
+elseif(XCODE_VERSION)
+  set(RELEASE_LIB_DIR Release)
+  set(DEBUG_LIB_DIR Debug)
+else()else()
   set(RELEASE_LIB_DIR "")
   set(DEBUG_LIB_DIR "")
 endif()