Userspace RCU

If multiple builds with different compile flags are desired for the same source tree, it's common to perform the build outside the source tree.

Steps to reproduce:

git clone git://git.lttng.org/userspace-rcu.git userspace-rcu
mkdir userspace-rcu-64bit
cd userspace-rcu
./bootstrap
cd ../userspace-rcu-64bit
../userspace-rcu/configure
make

Expected:
Proper compilation

Obtained:

make  all-recursive
make[1]: Entering directory `userspace-rcu-64bit'
Making all in .
make[2]: Entering directory `userspace-rcu-64bit'
  CC     wfqueue.lo
  CC     wfstack.lo
  CCLD   liburcu-common.la
  CC     urcu.lo
  CC     urcu-pointer.lo
  CCLD   liburcu.la
  CC     urcu-qsbr.lo
  CCLD   liburcu-qsbr.la
  CC     liburcu_mb_la-urcu.lo
  CC     liburcu_mb_la-urcu-pointer.lo
  CCLD   liburcu-mb.la
  CC     liburcu_signal_la-urcu.lo
  CC     liburcu_signal_la-urcu-pointer.lo
  CCLD   liburcu-signal.la
  CC     urcu-bp.lo
  CCLD   liburcu-bp.la
  CC     rculfqueue.lo
  CC     rculfstack.lo
  CCLD   liburcu-cds.la
make[2]: *** No rule to make target `urcu/map/*.h', needed by `all-am'.  Stop.
make[2]: Leaving directory `userspace-rcu-64bit'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `userspace-rcu-64bit'
make: *** [all] Error 2