Fix FreeBSD 10

Fix for FreeBSD 10, the regexp for parsing `ex.message` was lifted from: https://github.com/ffi/ffi/pull/328

That patch didn't quite work, though. This one should.

I did *not* test this on any other OS (ie. Linux), lacking access to such a system...

This fixes: https://github.com/ffi/ffi/issues/308

For reference, the contents of /usr/lib/libc.so is:

    /* $FreeBSD: release/10.0.0/lib/libc/libc.ldscript 258398 2013-11-20 20:24:59Z peter $ */
    GROUP ( /lib/libc.so.7 /usr/lib/libc_nonshared.a /usr/lib/libssp_nonshared.a )