Support for remote LIBOBJS (and friends) with subdir-objects.
Fixes PR automake/401.
* automake.in (config_libobj_dir): New variable.
(scan_autoconf_traces): Set config_libobj_dir from AC_CONFIG_LIBOBJ_DIR.
(handle_LIBOBJS_or_ALLOCA, require_libsource_with_macro): New functions.
(handle_LIBOBJS, handle_ALLOCA): Use them. Adjust location of
dependency files, possibly in a subdirectory.
* tests/pr401.test, tests/pr401b.test, tests/pr401c.test: New tests.
* tests/Makefile.am (TESTS): Add them.
* doc/automake.texi (Optional) <AC_CONFIG_LIBOBJ_DIR>: Document.
(LIBOBJS): Document changes in behaviour of
LIBOBJS, ALLOCA, LTLIBOBJS & LTALLOCA in the presence of
subdir-objects and an invocation of AC_CONFIG_LIBOBJ_DIR.