It was "ucru," but it should be "urcu". The pairwise CI job fails as a
result with:
configure: error: unrecognized options: --with-libucru
#
# Pick Userspace-RCU flavor (developer only option)
#
-# [pairwise: --with-liburcu=membarrier, --with-liburcu=qsbr, --with-libucru=mb, --with-liburcu=signal, --with-liburcu=bp]
+# [pairwise: --with-liburcu=membarrier, --with-liburcu=qsbr, --with-liburcu=mb, --with-liburcu=signal, --with-liburcu=bp]
AC_ARG_WITH([liburcu],
[AS_HELP_STRING([--with-liburcu=FLAVOR],
[Build with Userspace-RCU variant (membarrier|qsbr|mb|signal|bp) [default=membarrier]]))],