Autotools is still supported; use it, so that it remains well tested and at
feature parity with the CMake build system it also supports.
* gnu/packages/networking.scm (opendht)[build-system]: Switch to
gnu-build-system.
[arguments]: Move field above inputs.
[imported-modules, modules]: Adjust accordingly.
[configure-flags]: Likewise.
[phases]: Use G-Exp, and adjust accordingly.