The GNU Autotools are the venerable build system behind the familiar incantation ”./configure && make && make install.” The core piece, Autoconf, is described by the GNU project as “an extensible package of M4 macros that produce shell scripts to automatically configure software source code packages.” It is a GNU development tool maintained by volunteers at the Free Software Foundation.
The problem Autotools solved was portability. In the era when Autoconf emerged (autoconf’s early releases date to around 1991), Unix existed as many incompatible variants, and a C program that compiled on one might fail on another because of differing headers, libraries, and system calls. Autoconf reads a list of operating-system features a package can use, written as M4 macro calls, and generates a self-contained configure script that detects those features on the user’s machine and adapts the build automatically.
A key design point is that the generated configure scripts are standalone: end users do not need Autoconf or M4 installed to build a package, only the developers who create it do. Autoconf works alongside its sibling tools, GNU Automake (which generates portable Makefiles) and GNU Libtool (which abstracts the platform-specific details of building shared libraries).
Together these tools defined how open-source C software was packaged and distributed for decades, and they remain the standard build system for a large body of GNU and Unix software, even as newer generators such as CMake have taken over much of the C++ world.