obsolete replacements (that aren't always)

[Karl Berry]

Thank you for all the information in the Autoconf manual about replacing
"obsoleted" macros. A couple comments.

AC_FUNC_VFORK is warned about, but not mentioned in the manual
anywhere. Maybe it should be another @itemx with AC_VFORK.

AC_PROG_LEX is warned about when used without an option. But the
behavior with no options cannot be replicated with either option. So it
seems like it should not be considered obsolete? Even if the behavior
was "due to a bug". Such is reality.

AC_HEADER_TIME is complained about, but the additional #definition of
TIME_WITH_SYS_TIME makes it different than the replacement. Again this
seems like it is not obsolete as a result. I can't imagine many
maintainers would be happy spending time tinkering with time.h
vs. sys/time.h code that has worked forever just because there's a new
Autoconf macro.

Best,
Karl

P.S. Just FYI: with Autoconf 2.71 (unlike 2.70), TeX Live built again
without any changes to the configure.ac/Makefile.am/etc. files. Thanks!