[tlbuild] expand_depth vs. sigsegv check

[Karl Berry]

Anyone who has a chance, can you please try running
  tex '\def\z{\number\z}\z'
on the command line (or the equivalent)? The desired result is:
! TeX capacity exceeded, sorry [expansion depth=10000].

The question is whether that msg comes up, or whether TeX crashes with
a SIGSEGV or similar memory fault because the runtime stack is exhausted
before the expand_depth=10000 limit is hit. DavidC noticed this
happening on Cygwin, where the stack may be a bit smaller than "normal".

The amount of available memory is presumably a factor, so the "smaller"
the machine, the better. There's no point in trying the above on
GNU/Linux machines with reasonable memory sizes, since that's what I
have, and it doesn't crash there :).

Trying to figure out whether we should decrease the 10000 to something
else, or if it's specific to Cygwin. If you do get a memory crash
instead of the nice TeX message, please try decreasing expand_depth, as in
  env expand_depth=5000 tex '\def\z{\number\z}\z'

I'm doubtful we'd want to decrease it below 5000 in any case.
Just my feeling. --thanks, karl.