Add support for the libbacktrace library

[krader1961]

The libbacktrace library makes it possible to produce backtraces comparable to what you would see from gdb or lldb. Without having to shell out to commands like addr2line or atos like we currently do. This would make producing useful backtraces much more efficient. Not to mention make it possible to efficiently produce diagnostic output that needs to convert a pointer to a symbol name and file+line such as when printing a pointer to a ksh function that is embedded in a struct.

Only one of the distros I have setup for running ksh tests has that library installed by default. So we'll need to embed the libbacktrace source in this project.