In the GNU Build System, packages can be nested to arbitrary depth. This means that a package can embed other packages with their own configure, Makefiles, etc.
These other packages should just appear as subdirectories of their
parent package. They must be listed in SUBDIRS like other
ordinary directories. However the subpackage’s Makefiles
should be output by its own configure script, not by the
parent’s configure. This is achieved using the
AC_CONFIG_SUBDIRS Autoconf macro (see Configuring Other Packages in Subdirectories in The Autoconf Manual).
Here is an example package for an arm program that links with
a hand library that is a nested package in subdirectory
hand/.
arm’s configure.ac:
AC_INIT([arm], [1.0]) AC_CONFIG_AUX_DIR([.]) AM_INIT_AUTOMAKE AC_PROG_CC AC_CONFIG_FILES([Makefile]) # Call hand's ./configure script recursively. AC_CONFIG_SUBDIRS([hand]) AC_OUTPUT
arm’s Makefile.am:
# Build the library in the hand subdirectory first. SUBDIRS = hand # Include hand's header when compiling this directory. AM_CPPFLAGS = -I$(srcdir)/hand bin_PROGRAMS = arm arm_SOURCES = arm.c # link with the hand library. arm_LDADD = hand/libhand.a
Now here is hand’s hand/configure.ac:
AC_INIT([hand], [1.2]) AC_CONFIG_AUX_DIR([.]) AM_INIT_AUTOMAKE AC_PROG_CC AM_PROG_AR AC_PROG_RANLIB AC_CONFIG_FILES([Makefile]) AC_OUTPUT
and its hand/Makefile.am:
lib_LIBRARIES = libhand.a libhand_a_SOURCES = hand.c
When ‘make dist’ is run from the top-level directory it will
create an archive arm-1.0.tar.gz that contains the arm
code as well as the hand subdirectory. This package can be
built and installed like any ordinary package, with the usual
‘./configure && make && make install’ sequence (the hand
subpackage will be built and installed by the process).
When ‘make dist’ is run from the hand directory, it will create a self-contained hand-1.2.tar.gz archive. So although it appears to be embedded in another package, it can still be used separately.
The purpose of the ‘AC_CONFIG_AUX_DIR([.])’ instruction is to
force Automake and Autoconf to search for auxiliary scripts in the
current directory. For instance, this means that there will be two
copies of install-sh: one in the top-level of the arm
package, and another one in the hand/ subdirectory for the
hand package.
The historical default is to search for these auxiliary scripts in the
parent directory and the grandparent directory. So if the
‘AC_CONFIG_AUX_DIR([.])’ line was removed from
hand/configure.ac, that subpackage would share the auxiliary
script of the arm package. This may look like a gain in size
(a few kilobytes), but more importantly, it is a loss of modularity as
the hand subpackage is no longer self-contained (‘make
dist’ in the subdirectory will not work anymore).
Packages that do not use Automake need more work to be integrated this way. See Third-Party Makefiles.