Add workaround to prevent “libatomic not found” when building on debian against debian capnproto package which installs a buggy CapnProtoConfig.cmake file. The bug was introduced as part of debian’s attempt to fix a different issue: https://github.com/capnproto/capnproto/issues/1448 and their fix works for autoconf projects but not cmake projects like libmultiprocess.
Fixes #68