Companion library to liberror. It enables backtrace
support for liberror.

The library need only be linked, together with -ldw
(from libelf) and -lunwind (libunwind); there is no
header file or any functions the user should use.
Note, however, for static linking you may need
-Wl,--whole-archive before and -Wl,--no-whole-archive
after the library when linking; for dynamic linking,
put -lerror-backtrace before -lerror; alternatively,
the library can be loaded with LD_PRELOAD if liberror
is dynamically linked.