(And update liburcu to 0.8.4 according to release notes for lttng 2.4.x.) In addition to new features and bug fixes, version 2.4.x is needed to build against Linux 3.12 (our new stable kernel).
liburcu is a userspace RCU (read-copy-update) library that is used by LTTng - Linux Trace Toolkit (next generation).