Fix build on FreeBSD

Fix build on FreeBSD

__GLIBC_PREREQ is defined in such a way that the ! cannot be used in front of
it on FreeBSD.  Also, -lpthread is not implied by the build and we definitely
use it for compatfd support.

While at it, I added a default initialization for posix-aio that seems to
perform well in our testing.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5322 c046a42c-6fe2-441c-8c8c-71466251a162