regex: avoid warning when pointers are not long

regex: avoid warning when pointers are not long

* lib/regcomp.c (parse_dup_op, mark_opt_subexp): Cast between void *
and uintptr_t, not long, for portability to hosts where pointers and
long have different sizes.  Issue noted by Daniel P. Berrange in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-06/msg00122.html>
and fix suggested by Bruno Haible in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-06/msg00128.html>.