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 <[email protected]>
Co-authored-by: Illia Bobyr <[email protected]>

Author: illia-bobyr

runtime/doc/todo.txt

@@ -3536,8 +3536,6 @@ Macintosh:
 8   Dragging the status line doesn't scroll but redraw.
 8   When performing incremental search, should abort searching as soon as a
     character is typed.
-8   When the value of $MAKE contains a path, configure can't handle this.
-    It's an autoconf bug.  Remove the path from $MAKE to work around it.
 8   How to set VIMRC_FILE to \"something\" for configure?  Why does this not
     work: CFLAGS='-DVIMRC_FILE=\"/mydir/myfile\"' ./configure
 8   The temporary file is sometimes not writable.  Check for this, and use an

src/Makefile

@@ -627,7 +627,7 @@ CClink = $(CC)
 #EXTRA_LIBS = /usr/local/lib/libefence.a
 
 # Autoconf binary.
-AUTOCONF = autoconf
+AUTOCONF ?= autoconf
 
 # PURIFY - remove the # to use the "purify" program (hoi Nia++!)
 #PURIFY = purify
@@ -2014,9 +2014,6 @@ reconfig: scratch clean
 # - Use sed to change ./config.log to auto/config.log in the configure script.
 # Autoconf 2.5x (2.59 at least) produces a few more files that we need to take
 # care of:
-# - configure.lineno: has the line numbers replaced with $LINENO.  That
-#   improves patches a LOT, thus use it instead (until someone says it doesn't
-#   work on some system).
 # - autom4te.cache directory is created and not cleaned up.  Delete it.
 # - Uses ">config.log" instead of "./config.log".
 autoconf: