Add --enable-rcu-debug to configure

When used CONFIG_RCU_DEBUG is defined in urcu/config.h, thus the
debugging self-test are used at all time. This enables a permanent
built-in debugging behaviour.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>