(automake-1.16.info)Subdirectories with AM_CONDITIONAL

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7.2.2 Subdirectories with ‘AM_CONDITIONAL’
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‘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.