And last (and probably least) is the perl script that pulls the dependency stuff out of Makefile and into .depend.

eval '(exit $?0)' && eval 'exec perl -w -S $0 ${1+"$@"}'
    & eval 'exec perl -S $0 $argv:q'
    if 0;

require "getopts.pl";
&Getopts("f:o:");

$opt_f = "Makefile" if( ! $opt_f );
$opt_o = ".depend"  if( ! $opt_o );

  # Open the Makefile that has been mangled by 'make depend'
  # and suck it into a perl array.
open(IF,"<$opt_f") || die;
@makefile = <IF>;
close(IF);

  # Now open our .depend file and a temporary Makefile.
  # We'll split the original Makefile between these two.
open(DF,">$opt_o") || die;
open(MF,">$opt_f.tmp") || die;

  # For each line we read out of the original file...
foreach (@makefile) {

  # If we're into the dependency section, write the line
  # into the .depend file.
  #
  if( $depend ) {
    print DF $_;
  }
  else {
    # If we haven't gotten to the dependency section yet
    # then see if the current line is the separator that
    # "make depend" causes to be inserted.
    #
    if( m/^\Q# DO NOT DELETE THIS LINE -- g++dep uses it.\E/ ) {

      # If so, change our "mode" and skip this line.
      ++$depend;
      next;
    }

    # Also skip the "include .depend" that we insert.  If we
    # don't do this, it is possible to have a bunch of these
    # inserted into the output when we read an unmangled Makefile
    next if( m/^include $opt_o/ );

    # Print the non-dependency info to the temporary Makefile
    print MF $_;
  }
}

# Tell our new Makefile to include the dependency file
print MF "include $opt_o\n";

# Close the two output files...
close(DF);
close(MF);

# Unlink (remove) the original Makefile and rename our
# temporary file.  There's obviously room for error checking
# here but we've got the Makefile checked into some revision
# control system anyway.  Don't we?

unlink("$opt_f");
rename("$opt_f.tmp","$opt_f");

exit(0);