6.2.1 Conditional subdirectories with AM_CONDITIONAL

`configure' should output the `Makefile' for each directory and define a condition into which `opt/' should be built.

... AM_CONDITIONAL([COND_OPT], [test "$want_opt" = yes]) AC_CONFIG_FILES([Makefile src/Makefile opt/Makefile]) ...

Then SUBDIRS can be defined in the top-level `Makefile.am' as follows.

if COND_OPT MAYBE_OPT = opt endif SUBDIRS = src $(MAYBE_OPT)

As you can see, running make will rightly recurse into `src/' and maybe `opt/'.

As you can't see, running make dist will recurse into both `src/' and `opt/' directories because make dist, unlike make all, doesn't use the SUBDIRS variable. It uses the DIST_SUBDIRS variable.

In this case Automake will define DIST_SUBDIRS = src opt automatically because it knows that MAYBE_OPT can contain opt in some condition.