automake-1.16: Subdirectories with AC_SUBST
7.2.3 Subdirectories with ‘AC_SUBST’
------------------------------------
Another possibility is to define ‘MAYBE_OPT’ from ‘./configure’ using
‘AC_SUBST’:
...
if test "$want_opt" = yes; then
MAYBE_OPT=opt
else
MAYBE_OPT=
fi
AC_SUBST([MAYBE_OPT])
AC_CONFIG_FILES([Makefile src/Makefile opt/Makefile])
...
In this case the top-level ‘Makefile.am’ should look as follows.
SUBDIRS = src $(MAYBE_OPT)
DIST_SUBDIRS = src opt
The drawback is that since Automake cannot guess what the possible
values of ‘MAYBE_OPT’ are, it is necessary to define ‘DIST_SUBDIRS’.