In some C-libraries (like uclibc), backtrace support is optional, so the
execinfo.h may not exist.

This change adds the check for execinfo.h header and conditionaly enable
backtrace support.

This issue has been triggered by Buildroot farms:
  http://autobuild.buildroot.org/results/391/391e71a988250ea66ec4dbee6f60fdce9eaf2766/build-end.log

Signed-off-by: Samuel Martin <s.martin49@gmail.com>