buildroot

package/transmission: add optional support for libminiupnpc/libnatpmp

Transmission bundles copies of both libraries and uses them if they are
not provided by the system. This patch allows transmission to use the
system libraries of libminiupnpc/libnatpmp.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>