6.2 Conditional subdirectories

It is possible to define the SUBDIRS variable conditionally if, like in the case of GNU Inetutils, you want to only build a subset of the entire package.

To illustrate how this works, let's assume we have two directories `src/' and `opt/'. `src/' should always be built, but we want to decide in ./configure whether `opt/' will be built or not. (For this example we will assume that `opt/' should be built when the variable $want_opt was set to yes.)

Running make should thus recurse into `src/' always, and then maybe in `opt/'.

However make dist should always recurse into both `src/' and `opt/'. Because `opt/' should be distributed even if it is not needed in the current configuration. This means `opt/Makefile' should be created unconditionally. (3)

There are two ways to setup a project like this. You can use Automake conditionals (see section 20. Conditionals) or use Autoconf AC_SUBST variables (see section `Setting Output Variables' in The Autoconf Manual). Using Automake conditionals is the preferred solution.