compositor: Use libunwind if available for better backtraces

compositor: Use libunwind if available for better backtraces

libunwind has a dwarf parser and automatically queries the dlinfo
for location of dlopened modules.  The resulting backtrace is much
better and includes stack frames in dynamically loaded modules.

krh: Originally submitted for Xorg, adapted for weston:

  http://lists.x.org/archives/xorg-devel/2013-February/035493.html

Note this require libunwind at least 1.1 to get the pkg-config files.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>