cmSystemTools: Remove unreachable return on HP compiler

Follow up commit 1de08685 (cmSystemTools: Restore unreachable return to
silence warnings, 2014-04-02) by excluding the unreachable return on the
HP compiler as well as Clang.  Both recognize that it is unreachable and
warn.

Source/cmSystemTools.cxx

@@ -1616,7 +1616,7 @@ long copy_data(struct archive *ar, struct archive *aw)
       return (r);
       }
     }
-#if !defined(__clang__)
+#if !defined(__clang__) && !defined(__HP_aCC)
   return r; /* this should not happen but it quiets some compilers */
 #endif
 }