fix compilation with Centos 5.8

LifoSem and MemoryIdler use folly/detail/Futex.h (Futex had been disabled already) that don't compile at Centos 5.8 - Linux 2.6.18 , because FUTEX_WAIT_BITSET, FUTEX_WAKE_BITSET, FUTEX_CLOCK_REALTIME aren't defined