Make backtrace feature optional

[wyoung]

The backtrace(3) family of library functions don't exist under Cygwin. They are, in fact, GNU-specific, so they probably don't exist on several other platforms, either.

There are a bunch of ways to deal with this. I've attached a fairly brute-force removal of this on Cygwin through ifdefs, but perhaps something like what you see around line 23011 in libstuff/sqlite3.c would be better.