7.2.2 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.