Although nesting packages isn't something we would recommend to someone who is discovering the Autotools, it is a nice feature worthy of mention in this small advertising tour.
Autoconfiscated packages (that means packages whose build system have been created by Autoconf and friends) can be nested to arbitrary depth.
A typical setup is that package A will distribute one of the libraries it needs in a subdirectory. This library B is a complete package with its own GNU Build System. The configure script of A will run the configure script of B as part of its execution, building and installing A will also build and install B. Generating a distribution for A will also include B.
It is possible to gather several packages like this. GCC is a heavy user of this feature. This gives installers a single package to configure, build and install, while it allows developers to work on subpackages independently.
When configuring nested packages, the configure options given to the top-level configure are passed recursively to nested configures. A package that does not understand an option will ignore it, assuming it is meaningful to some other package.
The command configure --help=recursive can be used to display
the options supported by all the included packages.
See Subpackages, for an example setup.