scripts/realtime: catch common install error - forgot to 'sudo make setuid'
configure: detect tcl8.6/tk8.6
configure: detect installation of libbacktrace

this is either bundled from gcc-4.8 on, or can be custom-installed
via https://github.com/mhaberler/libbacktrace for earlier gcc versions.

If found, rtapi_app and rtapi_msgd create a symbolic backtrace with
line numbers in the log.
Make: add -funwind-tables to CFLAGS

This just creates static information for libbacktrace to work.
No runtime overhead. Seems to be default for later gcc versions, no harm
in activating.
backtrace: support code in libmtalk

see mk-backtrace.h for the API.
Usage examples in rtapi/rtapi_app.cc and rtapi/rtapi_msgd.cc .
rtapi_app,rtapi_msgd: symbolic backtrace support

conditional on libbacktrace being installed.
haltalk: add symbolic backtrace
Merge pull request #280 from mhaberler/libbacktrace

symbolic backtrace support for RT