Define AM_RECURSIVE_TARGETS, for gnulib's GNUmakefile.

Define AM_RECURSIVE_TARGETS, for gnulib's GNUmakefile.

This new macro lists all public targets which invoke `make'
recursively, or depend on targets which do so.  It allows to
prevent parallelism selectively, when multiple targets are
passed on the `make' command line.

* lib/am/distdir.am [%?SUBDIRS%] (AM_RECURSIVE_TARGETS): New
macro.
* lib/am/subdirs.am (AM_RECURSIVE_TARGETS): Likewise.
* lib/am/tags.am [%?SUBDIRS%] (AM_RECURSIVE_TARGETS):
Likewise.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>