thread_it.pl: Increase Mac stack

At Vincent’s suggestion (<5213ABBE.9080706@profvince.com>), increase
the Mac stack size to 1000000.
[perl #118753] Remove erroneous elsif("") warning

‘5 || foo’ gets optimised down to a plain ‘5’.  If that happens in
void context, we should not get a warning, as constant folding is not
supposed to change behaviour.

We accomplish that by flagging the constant with
OPpCONST_SHORTCIRCUIT.  The code that warns checks for the absence of
that flag first.

Commit f6f3a1fea, in order to improve line number accuracy in elsif,
changed the condition in elsif() to have a nulled COP (containing line
numbers) with the condition as its kidop.

That prevented constant folding from happening, so commit 71c4dbc37
implemented a function (search_const) to search to find a constant
below the nulled COP, but only used it to make folding happen, and did
not use it for OPpCONST_SHORTCIRCUIT.

The result was that if($foo){}elsif(""){} (mostly equivalent to
$foo ? do{} : "" && do {} internally) got folded to $foo ? do{} : ""
without OPpCONST_SHORTCIRCUIT being set on the final "".  So the warn-
ing occurs.

This commit uses search_const to set OPpCONST_SHORTCIRCUIT, and the
warning vanishes.