* doc/automake.texi (Requirements) <AC_CONFIG_FILES>: Discuss

* doc/automake.texi (Requirements) <AC_CONFIG_FILES>: Discuss
specifications with shell variables.
(Optional) <AC_CONFIG_HEADERS, AC_CONFIG_LINKS>: Point to
AC_CONFIG_FILES for this explanation.
* automake.in (substitute_ac_subst_variables_worker,
substitute_ac_subst_variables): Mew functions.
(rewrite_inputs_into_dependencies): Use substitute_ac_subst_variables
to ignore dependencies that contain unAC_SUBSTed shell variables.
(handle_configure): Likewise, do not output rules for
AC_CONFIG_HEADERS, AC_CONFIG_FILES, and AC_CONFIG_LINKS targets that
unAC_SUBSTed contain shell variables.
* tests/autohdr4.test: Use an AC_SUBST variable in a specification.
* tests/output11.test, tests/output12.test: New files.
* tests/Makefile.am (TESTS): Add output11.test and output12.test.