sysutils/liburcu: Update to 0.15.6 and return to pool

Add CONFIGURE_ENV to avoid building with debug symbols which we later
discard
Return to pool due to multiple timeouts

Backport in following upstream commits:
16b21287ee99c4f79d49f1840dddb8cc050abee1
ca541c6b8c6ce1c166329fa1c19345d8f7794a5a
5c33a80edb016c2241cebabc07fc42698ee5d513
f240cba1dbadd0649f523953cd53fada508d82b5
d5ffaf362824a7cfeda8c7f0479bfb1c7688e65d
1f52ea43e6c0c07bd77886ef762a14e37fd42f89
7eb03ba629b3205696efaefaa51dd37b519ac128

Changelog: https://github.com/urcu/userspace-rcu/blob/v0.15.6/ChangeLog

References:
https://github.com/urcu/userspace-rcu/commit/16b21287ee99c4f79d49f1840dddb8cc050abee1
https://github.com/urcu/userspace-rcu/commit/ca541c6b8c6ce1c166329fa1c19345d8f7794a5a
https://github.com/urcu/userspace-rcu/commit/5c33a80edb016c2241cebabc07fc42698ee5d513
https://github.com/urcu/userspace-rcu/commit/f240cba1dbadd0649f523953cd53fada508d82b5
https://github.com/urcu/userspace-rcu/commit/d5ffaf362824a7cfeda8c7f0479bfb1c7688e65d
https://github.com/urcu/userspace-rcu/commit/1f52ea43e6c0c07bd77886ef762a14e37fd42f89
https://github.com/urcu/userspace-rcu/commit/7eb03ba629b3205696efaefaa51dd37b519ac128

PR: 296786, 293737
Reported by: Ondřej Surý, Borja Marcos
Tested by: Borja Marcos