usleep is deprecated and disabled in glibc 2.12 unless requested. Use
nanosleep instead if available.
This fixes the following compiler warning:
./src/zrtp_iface_scheduler.c: In function 'zrtp_sleep':
./src/zrtp_iface_scheduler.c:96:2: warning: implicit declaration of
function 'usleep' [-Wimplicit-function-declaration]
usleep(msec*1000);
^