build-sys: link with slirp as an external project

Use the "system" libslirp if its present or requested.

Else build with a static libslirp.a if slirp/ is checked
out ("internal") or a submodule ("git").

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190212162524.31504-7-marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>