INN requires several other packages be installed in order to be fully functional (or in some cases, to work at all):
In order to build INN, you will need a C compiler that understands ANSI C. If you are trying to install INN on an operating system that doesn't have an ANSI C compiler (such as SunOS), installing gcc is recommended. You can get it from ftp://ftp.gnu.org/gcc/ or its mirrors. INN is tested with gcc more thoroughly than with any other compiler, so even if you have another compiler available, you may wish to use gcc instead.
INN requires Perl to build and to run several subsystems. Though most of those components are compatible with Perl 4, some features (such as script embedded in the server) mandate Perl 5. Effort is made to ensure that all parts of INN will work with Perl 5.004; older versions may or may not be adequate. It's therefore recommended that you have at least Perl 5.004 installed before attempting to compile INN. For instructions on obtaining and installing Perl, see http://www.perl.com/latest/.
The INN Makefiles use the syntax include FILE, rather than the syntax common on BSDish systems of .include <FILE>. If your system expects the BSDish syntax, you can address this in two ways: change each Makefile's include lines, or install GNU make (ftp://ftp.gnu.org/make/). If you choose the latter route, make sure that the path to make in site/do-subst.sh points to GNU make.
FIXME: Is this reference to site/do-subst.sh still needed? Is there any way we can make autoconf handle this for us, including putting the right make program in site/do-subst.sh? (Is anyone still using subst or this script now that the build system isn't any more?)
If you want to enable support for authenticated control messages (this is NOT required) then you will need to install PGP. Unfortunately, the licensing restrictions on PGP are extremely unclear; it's possible that if you are installing INN for a company, even if the news server is not part of the business of that company, you would need to purchase a commercial license for PGP. For an educational or non-profit organization, this shouldn't be a problem. See http://www.pgp.com/ for more information about PGP, including the licensing restrictions.
You may also want to look at GnuPG (http://www.gnupg.org/), a free software OpenPGP implementation. Unfortunately, most control message signatures are currently PGP 2.6.2-compatible signatures, which cannot be handled by stock GnuPG for patent reasons. However, there are add-on modules for GnuPG that allow verification of those types of signatures, particularly if you are outside the United States.
Also, if you want to use either the Tcl or Python embedded hooks, you'll need to have suitable versions of Tcl and/or Python installed. See README.tcl_hook and README.python_hook for more information.