What does MAIN_ _MAIN_ __MAIN__ mean?(to invoke clapack)

From: Peng Yu (pengyu.ut_at_gmail.com)
Date: 10/29/04 

Date: Fri, 29 Oct 2004 13:24:04 -0500

At http://www.netlib.org/clapack/faq.html, you will see some code like
the following. Do you know why?

void MAIN_(){}
void MAIN__(){}
void _MAIN_(){}