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Automake considers two sets of directories, defined by the variables SUBDIRS and DIST_SUBDIRS.

SUBDIRS contains the subdirectories of the current directory that must be built (see Subdirectories). It must be defined manually; Automake will never guess a directory is to be built. As we will see in the next two sections, it is possible to define it conditionally so that some directory will be omitted from the build.

DIST_SUBDIRS is used in rules that need to recurse in all directories, even those that have been conditionally left out of the build. Recall our example where we may not want to build subdirectory opt/, but yet we want to distribute it? This is where DIST_SUBDIRS comes into play: ‘opt’ may not appear in SUBDIRS, but it must appear in DIST_SUBDIRS.

Precisely, DIST_SUBDIRS is used by ‘make maintainer-clean’, ‘make distclean’ and ‘make dist’. All other recursive rules use SUBDIRS.

If SUBDIRS is defined conditionally using Automake conditionals, Automake will define DIST_SUBDIRS automatically from the possible values of SUBDIRS in all conditions.

If SUBDIRS contains AC_SUBST variables, DIST_SUBDIRS will not be defined correctly because Automake does not know the possible values of these variables. In this case DIST_SUBDIRS needs to be defined manually.