Autoconf is a tool for producing shell scripts that automatically configure software source code packages to adapt to many kinds of UNIX-like systems. The configuration scripts produced by Autoconf are independent of it when they are run.
Autoconf makes use of GNU m4 to transform a user-written 'configure.ac' (formerly, and on older projects, called 'configure.in') file to a portable shell script called 'configure'. The 'configure' script runs non-interactively, and generates customized headers and makefiles derived from pre-written templates. You could say that Autoconf compiles the M4 program in 'configure.ac' into a shell script.
Autoconf was begun in the summer of 1991 by David Mackenzie to support his work at the Free Software Foundation. In the subsequent years it grew to include enhancements from a variety of authors and became the most widely used build configuration system for writing portable free or open source software.
The Autoconf approach to portability is to test for features, not for versions. For example, the native C compiler on SunOS 4 did not support ISO C. However, it is possible for the user or administrator to have installed an ISO C compliant compiler. A pure version-based approach would not detect the presence of the ISO C compiler, but a feature testing approach would be able to discover the ISO C compiler the user had installed. The rationale of this approach is to gain the following advantages: