#37 iaxclient fails to build on OpenBSD

[Sebastian Reitenbach]

compilation of iaxclient library fails on OpenBSD in lib/unixfuncs.c due to missing sched_get_priority_max and sched_get_priority_min.
Both values should be assumed to be 0 on OpenBSD.

Appended harsh patch fixes the problem for me, but its only useful to demonstrate the problem.

[Sebastian Reitenbach]

patch that demonstrates problem