Update to config.guess 2018-05-19 and config.sub 2018-05-24 from
git://git.savannah.gnu.org/config.git revision
3a2a927f547ee478147008c3fda2adb8a2b2ebc4.
This brings in musl support.
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.