Fix compilation when using system libminiupnpc with libutp disabled

If transmission uses its own copy of libminiupnpc in third-party/ then
the configure script adds that path to CFLAGS:
https://trac.transmissionbt.com/browser/trunk/configure.ac#L455

This path is also added if transmission uses its own copy of libutp:
https://trac.transmissionbt.com/browser/trunk/configure.ac#L349

But if a system-provided libminiupnpc is used and utp-support is
disabled using --disable-utp the result is a build error, detected by
buildroot autobuilders:
http://autobuild.buildroot.net/results/b79/b79e51140cb0320554a58ce59dcb33f53cd9211a/build-end.log

net.c:40:24: fatal error: libutp/utp.h: No such file or directory

Patch proposed by Yann E. MORIN:
http://lists.busybox.net/pipermail/buildroot/2016-July/167896.html

Patch sent upstream: https://trac.transmissionbt.com/ticket/6163

Signed-off-by: Bernd Kuhls <berndkuhls@hotmail.com>

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	2016-07-31 12:13:40.921660078 +0200
@@ -358,6 +358,10 @@
     else
       AC_MSG_ERROR("Unable to build uTP support -- C++ compiler not found")
     fi
+else
+    # For the system libminiupnpc with UTC disabled,
+    # point to our bundled UTP headers
+    LIBUTP_CFLAGS="-I\$(top_srcdir)/third-party/"
 fi
 AC_SUBST(LIBUTP_CFLAGS)
 AC_SUBST(LIBUTP_LIBS)
@@ -457,7 +461,7 @@
     LIBUPNP_LIBS_QT="\$\${TRANSMISSION_TOP}/third-party/miniupnp/libminiupnp.a"
 else
     AC_DEFINE([SYSTEM_MINIUPNP])
-    LIBUPNP_CFLAGS="" 
+    LIBUPNP_CFLAGS="${LIBUTP_CFLAGS}"
     LIBUPNP_LIBS="-lminiupnpc"
     LIBUPNP_LIBS_QT="-lminiupnpc"
 fi