CFLAGS/LDFLAGS have debug and sanitizers flags, which should be passed
to slirp compilation.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190424110041.8175-2-marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>