Fix and document rules to not touch the tree with `make -n'.

Fix and document rules to not touch the tree with `make -n'.

* doc/automake.texi (Multiple Outputs): Document the problem of
modifications during dry-run execution, propose solution.
* NEWS: Update.
* automake.in (lang_vala_finish_target): Split recipe so the
stamp file is not removed with GNU `make -n'.
(lang_yacc_target_hook): Separate removal of parser output file
and header remaking.
* lib/am/lisp.am ($(am__ELCFILES)): Determine whether -n was
passed to make, take care not to remove any files in that case.
* lib/am/remake-hdr.am (%CONFIG_H%): Separate removal of
%STAMP% file from induced remaking of config header.
* tests/autohdrdry.test, tests/lispdry.test, tests/yaccdry.test:
New tests.
* tests/Makefile.am (TESTS): Update.

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