Code under HAVE_STACK_LIMIT has no effect.

HAVE_STACK_LIMIT definition was commented out in the original Mercurial repo:
https://code.google.com/p/vim/source/detail?r=7e70fc748752

...and then completely removed:
https://code.google.com/p/vim/source/detail?r=6768ebd0bc04

But the code depending on it was left in.