HAVE_POSIX_CLOCKS doesn't imply CLOCK_BOOTTIME.

HAVE_POSIX_CLOCKS imples clock_gettime,settime and
CLOCK_REALTIME / CLOCK_MONOTONIC.

CLOCK_BOOTTIME appears only on newer versions of glibc
and might only be supported on newer kernels.

Change-Id: I66e724a3593538c3b80de2c5f81a964d3fa96eaf