The gnu project's tool that configures a source code distribution to compile and run on a different platform.

Among open source hackers, a mere running binary of a program is not considered a full release; what's interesting is a source tree that can be built into binaries using standard tools. Since the mid-1990s, autoconf, automake, and libtools have been the standard way to make a distribution portable so that it can be built on multiple operating systems without change.