Add Autotools, the GNU build system (`automake`, `autoconf`,
`libtool`). There is no specific page for it, so the introduction
to "the system" is as part of the `automake` manual.
This commit adds `libnl` Netlink library to the networking section,
and points the link to the offical page of `libnl` (github repository
links on that page).
Since `czmq`, which are ZeroMQ C high level bindings, is present
in the list, make sure `libzmq` core library, with a pure C API but
C++ backend is also present in the list.
Marc Sune