meson/configure.ac: pthread-stubs not present on OpenBSD

pthread-stubs is no longer required on OpenBSD and has been removed.
libpthread parts involved moved to libc.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
[Eric: add meson equivalent]
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>