Use cmake's FindBacktrace module to find backtrace(3)

Summary:
Add Backtrace_LIBRARY to the linked libraries, and
Backtrace_INCLUDE_DIRS to the header directories.

This should fix the builds on FreeBSD

Reviewers: FreeBSD, pino, bcooksley, yurchor

Reviewed By: bcooksley

Differential Revision: https://phabricator.kde.org/D26851