configure: Rewrite implementation of --update.

Previously --update was kind of a hack, as it just dumped all
the variables from BuildConfig back into ./configure and attempted
to rerun from there.

Instead, now we store all of the variables and environs configure
was invoked with in the top of BuildConfig and re-call it using them.
This is much more robust, as configure will actually recompute
all of the values from the BuildConfig.

As a side effect, if configure was originally invoked to build crosstools,
it would have done so again, so instead add a check to see if they
exist, and skip them if they do. It's easy enough to delete the
directory if you want to force a crosstools rebuild.

Briefly discussed with mmlr on IRC.

build/scripts/convert_build_config_to_shell_format.pl

@@ -1,23 +0,0 @@
-#!/usr/bin/perl
-
-use warnings;
-use strict;
-
-=head1 convert_build_config_to_shell_format
-
-This simple script converts its standard input from the jam-specific format
-that is being used in BuildSetup into a format that's understood by sh.
-
-=cut
-
-my $data;
-while (my $line = <>) {
-	$data .= $line;
-	if ($data =~ m{\s*([+\w]+)\s*\?=\s*\"?([^;]*?)\"?\s*;}gms) {
-		my ($variable, $value) = ($1, $2);
-		$variable =~ tr{+}{X};	# '+' is illegal as part of shell variable
-		print "$variable='$value'\n";
-		$data = '';
-	}
-}
-