bug#46578: 28.0.50; configure.ac warnings with autoconf 2.71

[Andy Moreton]

In GNU Emacs 28.0.50 (x86_64-w64-mingw32)
Repository revision: cead0ea38e3e15a544f7374b2e831623bda37f1d
Repository branch: master

Building from a clean tree, I observe the following warnings:

$ ./autogen.sh
Checking whether you have the necessary tools...
(Read INSTALL.REPO for more details on building Emacs)
Checking for autoconf (need at least version 2.65) ... ok
Your system has the required tools.
Building aclocal.m4 ...
Running 'autoreconf -fi -I m4' ...
configure.ac:1770: warning: Checking for headers with the preprocessor is
configure.ac:1770: deprecated. Specify prerequisite code to AC_CHECK_HEADER
configure.ac:1770: instead of using fourth argument `-'. (Many headers need
configure.ac:1770: no prerequisites. If you truly need to test whether
configure.ac:1770: something passes the preprocessor but not the compiler,
configure.ac:1770: use AC_PREPROC_IFELSE.)
../autoconf-2.71/lib/autoconf/headers.m4:115: _AC_CHECK_HEADER_PREPROC 
is expanded from...
../autoconf-2.71/lib/autoconf/headers.m4:56: AC_CHECK_HEADER is expanded 
from...
../autoconf-2.71/lib/autoconf/headers.m4:217: AC_CHECK_HEADERS is 
expanded from...
configure.ac:1770: the top level
configure.ac:1771: warning: The macro `AC_HEADER_TIME' is obsolete.
configure.ac:1771: You should run autoupdate.
../autoconf-2.71/lib/autoconf/headers.m4:743: AC_HEADER_TIME is expanded 
from...
configure.ac:1771: the top level
configure.ac:4719: warning: The macro `AC_TRY_LINK' is obsolete.
configure.ac:4719: You should run autoupdate.
../autoconf-2.71/lib/autoconf/general.m4:2920: AC_TRY_LINK is expanded 
from...
configure.ac:4719: the top level
You can now run './configure'.

Perhaps some adjustment is needed for autoconf 2.71.

    AndyM