Add a simple Makefile to build libslirp.a, a static library version of
libslirp, to be used by QEMU during a transition period, until a
shared library is available.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190212162524.31504-6-marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>