[conf] Add config variable to disable crash related overrides.

 - We do some nasty things w.r.t. installing or overriding signal handlers in
   order to improve our crash recovery support or interaction with crash
   reporting software, and those things are not necessarily appropriate when
   LLVM is being linked into a client application that has its own ideas about
   how to do things. This gives those clients a way to disable that handling at
   build time.

 - Currently, the code this guards is all Apple specific, but other platforms
   might have the same concerns so I went for a more generic configure
   name. Someone who is more familiar with library embedding on Windows can
   handle choosing which of the Windows/Signals.inc behaviors might make sense
   to go under this flag.

 - This also fixes the proper autoconf'ing of ENABLE_BACKTRACES. The code
   expects it to be undefined when disabled, but the autoconf check was just
   defining it to 0.