It is a truth universally acknowledged, that as a developer in possession of a new package, you must be in want of a build system.
In the Unix world, such a build system is traditionally achieved using the command make (see Overview). You express the recipe to build your package in a Makefile. This file is a set of rules to build the files in the package. For instance the program prog may be built by running the linker on the files main.o, foo.o, and bar.o; the file main.o may be built by running the compiler on main.c; etc. Each time make is run, it reads Makefile, checks the existence and modification time of the files mentioned, decides what files need to be built (or rebuilt), and runs the associated commands.
When a package needs to be built on a different platform than the one
it was developed on, its Makefile usually needs to be adjusted.
For instance the compiler may have another name or require more
options. In 1991, David J. MacKenzie got tired of customizing
Makefile for the 20 platforms he had to deal with. Instead, he
handcrafted a little shell script called configure to
automatically adjust the Makefile (see Genesis). Compiling his package was now
as simple as running ./configure && make.
Today this process has been standardized in the GNU project. The GNU
Coding Standards (see The Release Process) explains how each package of the
GNU project should have a configure script, and the minimal
interface it should have. The Makefile too should follow some
established conventions. The result? A unified build system that
makes all packages almost indistinguishable by the installer. In its
simplest scenario, all the installer has to do is to unpack the
package, run ./configure && make && make install, and repeat
with the next package to install.
We call this build system the GNU Build System, since it was grown out of the GNU project. However it is used by a vast number of other packages: following any existing convention has its advantages.
The Autotools are tools that will create a GNU Build System for your package. Autoconf mostly focuses on configure and Automake on Makefiles. It is entirely possible to create a GNU Build System without the help of these tools. However it is rather burdensome and error-prone. We will discuss this again after some illustration of the GNU Build System in action.