Bugfix and improvement for autogen.sh

Hey,

I was doing some compiling yesterday, and noticed that "autogen.sh
--help" clobbers your autogen.lastrun.

My original thought was to change the code to fix the bug, but then I
had an idea - make "--help" more useful.

Thus, my idea is to add this to autogen.sh:

my $pos = 0;
for my $arg (@ARGV) {
    if ($arg eq '--help') {
        system ("./help.sh " . $ARGV[$pos+1]);
        exit;
    }
    $pos++;
}

and also add the attached help.sh file.

Basically, it shows a short description for "autogen.sh --help", with
a link to the wiki page and general overview. but you can also do
"autogen.sh --help full" for everything, and "autogen.sh --help speed"
for specific instructions about speed boosting. We can add additional
sections to help.sh if we wanted to - perhaps notes re cgywin or other
helpful docs.

If no one has any objections, I'll push to master.

Cheers,
Josh

Attachment: help.sh
Description: Bourne shell script