macOS build fails because timer_create is missing

timer_create() became mandatory in 04e53271

The build now fails on macOS 10.4 as a result.

The code still contains conditionals for HAVE_TIMER_CREATE in ntpd/ntp_sandbox.c and ntpd/ntp_timer.c. Did we intend to make this function mandatory in wscript?

/cc @hal.murray