OpenAL needs -pthread

On FreeBSD (and maybe NetBSD as well), OpenAL needs -pthread in 
CFLAGS/LDFLAGS:

configure:2352: gcc -o conftest  -I/usr/local/include   -L/usr/local/lib 
conftest.c -lopenal   >&5
/usr/local/lib/libopenal.so: undefined reference to `pthread_create'
/usr/local/lib/libopenal.so: undefined reference to `pthread_attr_init'
/usr/local/lib/libopenal.so: undefined reference to `pthread_exit'

This should best be solved during configure instead of setting this 
globally. Also, this probably needs to be propagated into OpenAL.
cabal for linking a resulting application (although already the RTS 
should pull in -pthread).