FindThreads: fix printing a pointer value in test code

This causes a warning in C mode, and entirely fails in C++ mode:

CMake/Modules/CheckForPthreads.c: In function ‘runner’:
CMake/Modules/CheckForPthreads.c:34:27: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
      printf("%d CC: %d\n", (int)args, cc);
                            ^
Use %p to print out a pointer value, which will not cause any problems.

Modules/CheckForPthreads.c

@@ -31,7 +31,7 @@ void* runner(void* args)
   int cc;
   for ( cc = 0; cc < 10; cc ++ )
     {
-    printf("%d CC: %d\n", (int)args, cc);
+    printf("%p CC: %d\n", args, cc);
     }
   res ++;
   return 0;