This adds DWARF 5 support as well as an enhanced testsuite.
Patch assembled by Than McIntosh.
Add required support files to the config subdirectory.
Update dtest to be based on a statically linked program.
Includes (among other things) support for compressed debug sections,
a variety of bugfixes, and expanded test coverage.
This is a standalone version of the libbacktrace library that I
originally wrote for GCC. This is a copy of libbacktrace from GCC
trunk, with all dependencies incorporated here.