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The top-level `Makefile.am'
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   In non-flat packages, the top level `Makefile.am' must tell Automake
which subdirectories are to be built.  This is done via the `SUBDIRS'
variable.

   The `SUBDIRS' macro holds a list of subdirectories in which building
of various sorts can occur.  Many targets (e.g. `all') in the generated
`Makefile' will run both locally and in all specified subdirectories.
Note that the directories listed in `SUBDIRS' are not required to
contain `Makefile.am's; only `Makefile's (after configuration).  This
allows inclusion of libraries from packages which do not use Automake
(such as `gettext').  The directories mentioned in `SUBDIRS' must be
direct children of the current directory.  For instance, you cannot put
`src/subdir' into `SUBDIRS'.

   In a deep package, the top-level `Makefile.am' is often very short.
For instance, here is the `Makefile.am' from the GNU Hello distribution:

     EXTRA_DIST = BUGS ChangeLog.O README-alpha
     SUBDIRS = doc intl po src tests

   It is possible to override the `SUBDIRS' variable if, like in the
case of GNU `Inetutils', you want to only build a subset of the entire
package.  In your `Makefile.am' include:

     SUBDIRS = @SUBDIRS@

   Then in your `configure.in' you can specify:

     SUBDIRS = "src doc lib po"
     AC_SUBST(SUBDIRS)

   The upshot of this is that Automake is tricked into building the
package to take the subdirs, but doesn't actually bind that list until
`configure' is run.

   Although the `SUBDIRS' macro can contain configure substitutions
(e.g. `@DIRS@'); Automake itself does not actually examine the contents
of this variable.

   If `SUBDIRS' is defined, then your `configure.in' must include
`AC_PROG_MAKE_SET'.

   The use of `SUBDIRS' is not restricted to just the top-level
`Makefile.am'.  Automake can be used to construct packages of arbitrary
depth.

   By default, Automake generates `Makefiles' which work depth-first
(`postfix').  However, it is possible to change this ordering.  You can
do this by putting `.' into `SUBDIRS'.  For instance, putting `.'
first will cause a `prefix' ordering of directories.