Make NO_STACK_TRACE handling better.

as we discussed at pull request #21, im in a way to support stack trace on cygwin.
here is first step, for making `NO_STACK_TRACE` handling better.

note, even w/ this commits, it still has no real stack trace support on cygwin.
at least, i should add `cygwin_stackdump` to appropriated location (maybe, along w/ `#ifdef HAVE_EXECINFO_H`?),
and make cygwin-stackdump-example for proof of working.

IMPORTANT NOTE, ive tested this on cygwin only ATM, test to ensure no break on other platform is required.

any thoughts?