automake: update to 1.16.4.

New in 1.16.4:

* New features added

  - The PYTHON_PREFIX and PYTHON_EXEC_PREFIX variables are now set from
    Python's sys.prefix and sys.exec_prefix; use the new configure options
    --with-python_prefix and --with-python_exec_prefix to specify explicitly.

  - Common top-level files can be provided as .md; the non-md version is
    used if both are present:
      AUTHORS ChangeLog INSTALL NEWS README README-alpha THANKS

  - CTAGS, ETAGS, SCOPE variables can be set via configure.

  - Silent make output for custom link commands.

  - New option "no-dist-built-sources" skips generating $(BUILT_SOURCES)
    before building the tarball as part of "make dist", that is,
    omits the dependency of $(distdir): $(BUILT_SOURCES).

* Bugs fixed

  - automake output more reproducible.

  - test-driver less likely to clash with tests writing to the same file.

  - DejaGnu tests always use the directory name, testsuite/, for
    compatibility with the newer dejagnu-1.6.3 and with prior versions.

* Distribution

  - config.sub and config.guess updates include restoration of `...`
    for maximum portability.

(wiz)