Support for multiple inputs in AC_CONFIG_FILES.

Support for multiple inputs in AC_CONFIG_FILES.
Requested long ago by Harlan Stenn.
* automake.in (handle_dist): Remove unused argument $makefile.
(handle_subdirs): Expect SUBDIRS elements in $relative_dir,
not $am_relative_dir.
(rewrite_inputs_into_dependencies): Allow $add_srcdir to
be a filename that must always be prefixed by $(srcdir) or
$(top_srcdir).
(handle_configure, generate_makefile): Revamp to make the
Makefile.am, Makefile.in, Makefile more independent.
(locate_am): New function.
(scan_configure_dep, parse_arguments): Use locate_am.
(MAIN): Adjust call to generate_makefile.
* lib/am/configure.am: Remove %MAKEFILE-IN% from the dependencies
of %MAKEFILE%.  %MAKEFILE-IN% is already in %MAKEFILE-DEPS%.
* tests/output6.test, tests/output7.test: New files.
* tests/Makefile.am (TESTS): Add them.
* automake.texi (Requirements) <AC_CONFIG_FILES>: Document how
multiple inputs are scanned.