Complex sentence: Autoconf was begun in the summer of 1991 by David Mackenzie to support his work at the Free Software Foundation and later adopted enhancements from a variety of authors to become the most popular build configuration system for writing portable free or open source software.
Split into simpler sentences: 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.