patch 9.0.1823: Autoconf 2.69 too old

Problem:  Autoconf 2.69 too old
Solution: Migrate to Autoconf 2.71

Autoconf 2.69 is almost 10 years old.  And 2.71 is also a few years old
as well.  Should be pretty well tested by now.  It brings a lot of
improvements and there seems to be an ongoing work on autoconf 2.72
already.

This change just addresses two minor changes `autoupdate` suggested, and
then `src/auto/configure` is regenerated by running

  cd src
  make AUTOCONF=autoconf2.71 autoconf

closes: #12958

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Illia Bobyr <illia.bobyr@gmail.com>
diff --git a/src/configure.ac b/src/configure.ac
index 445ab21..1a4dd5c 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -1,8 +1,6 @@
 dnl configure.ac: autoconf script for Vim
 
-dnl Process this file with autoconf 2.69 to produce "configure".
-dnl This should also work with other versions of autoconf, but 2.70 and later
-dnl generate lots of hard to fix "obsolete" warnings.
+dnl Process this file with autoconf 2.71 to produce "configure".
 
 AC_INIT
 AC_CONFIG_SRCDIR([vim.h])
@@ -13,7 +11,7 @@
 AC_PROG_MAKE_SET
 
 dnl Checks for programs.
-AC_PROG_CC_C99		dnl required by almost everything
+AC_PROG_CC		dnl required by almost everything
 AC_PROG_CPP		dnl required by header file checks
 AC_PROG_EGREP		dnl required by AC_EGREP_CPP
 AC_PROG_FGREP		dnl finds working grep -F