Autobuild is a framework for building packages and for managing the dependencies of a package on other packages. It provides a common interface to configuring and building any package, but it is not a build system like make or cmake. You will still need platform-specific make, cmake, or project files to configure and build your library. Autobuild will, however, allow you invoke these commands and package the product with a common interface.
Important: Linden Lab Autobuild is not the same as or derived from GNU Autobuild, but they are similar enough to cause confusion.
For more information, see Autobuild's wiki page.