Fix compile error against miniupnpc-1.6 when --enable-upnp

Fix compile error against miniupnpc-1.6 when --enable-upnp

The bump from miniupnpc-1.5 to 1.6 changes the definition of
two functions used by tor-fw-helper-upnp.c, upnpDiscover() and
UPNP_AddPortMapping().  This patch addresses this and adds a
check in configure.in for backwards compatibility.

Thanks to Nickolay Kolchin-Semyonov for some hints.

X-Tor-Bug-URL: https://trac.torproject.org/projects/tor/ticket/5434
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=376621
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>