libtorrent: bump to version 0.13.6

libtorrent: bump to version 0.13.6

- Bump to version 0.13.6.

- Add a hash file.

- Change the website URL to the new official one.

- Help the build system to find the zlib headers, otherwise the
  configure script will fail like this:

    checking if zlib is wanted... yes
    checking for inflateEnd in -lz... no
    checking zlib.h usability... no
    checking zlib.h presence... no
    checking for zlib.h... no
    configure: error: either specify a valid zlib installation with
    --with-zlib=DIR or disable zlib usage with --without-zlib

- Disable instrumentation support by default since it avoids runtime
  failures on rtorrent for some architectures like MIPS, PowerPC or ARM:

    https://github.com/rakshasa/rtorrent/issues/156

- Tweak the 0001-cross_compile.patch for this new version.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>