updated for version 7.3.668

Problem:    Building with Perl loaded dynamically  still uses static library.
Solution:   Adjust use of PL_thr_key. (Ken Takata)

src/version.c

@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    668,
 /**/
     667,
 /**/