HAL usleep compile error, function not found

I'm developing a standalone system (no OS), using the NIOS2 hardware abstraction layer (HAL). Calls to usleep fail compilation - undefined reference.

My project folders show "altera components" with "Newlib C Library". I've included <unistd.h>. What have I missed?

NIOS2 IDE Version: 9.0SP2 Build: 235