Link with -lpthread on OpenBSD

[Adam Jensen]

Installation of [GSLL][1] on OpenBSD5.3 fails with the error:

* (ql:quickload "gsll")
sbcl:/usr/local/lib/libffi.so.6.1: undefined symbol 'pthread_mutex_unlock'
sbcl:/usr/local/lib/libffi.so.6.1: undefined symbol 'pthread_mutex_lock'
sbcl:/usr/local/lib/libffi.so.6.1: undefined symbol 'pthread_mutex_init'

I suspect this might be because libffi needs to be linked with -lpthread on OpenBSD.

A seperate issue - OpenBSD names libffi:

/usr/local/lib/libffi.so.0.0

[1]: http://common-lisp.net/project/gsll/