possible to disable zsh compatibility mode?

[Paul Lew <lew@xxxxxxxxxxxxxxxxxx>]

Is it possible to disable the sh/ksh compatibility mode in zsh?  My OS
(Digital UNIX) script program fork the shell as:

	 execl(shell, "sh", "-is", 0);

The shell came from environment variable SHELL, but the arg0 is 'sh'
which trigger compatibility mode.  When it entered the script zsh,
none of the rc files are read.  I have been manually run it so far.

I can either build my on script executable or modify zsh but I would
rather use a solution that will not require patch either program in
the future.  Any idea?  How about use another environment varaible?