Fix libminiupnpc version detection

Problem is known upstream: https://trac.transmissionbt.com/ticket/6140

For buildroot we know the libminiupnpc version so we hard-code 2.0.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr transmission-2.92.org/configure.ac transmission-2.92/configure.ac
--- transmission-2.92.org/configure.ac	2016-03-06 21:23:45.336198393 +0100
+++ transmission-2.92/configure.ac	2016-07-02 21:48:13.452385686 +0200
@@ -386,18 +386,7 @@
 dnl MINIUPNPC_API_VERSION and we won't have to figure
 dnl it out on our own
 if test "x$upnp_version" = "xunknown" ; then
-  AC_RUN_IFELSE(
-    [AC_LANG_PROGRAM(
-      [#include <stdlib.h>
-       #include <miniupnpc/miniupnpc.h>],
-      [#ifdef MINIUPNPC_API_VERSION
-       return EXIT_SUCCESS;
-       #else
-       return EXIT_FAILURE;
-       #endif]
-    )],
-    [upnp_version=">= 1.7"]
-  )
+  upnp_version="2.0"
 fi
 
 dnl Or maybe it's miniupnp 1.6