getting cc to recognize alloca

[ENGLUND TODD CHARLES]

     Sorry if this is a FAQ, but does anyone know which flags to
give cc so that it will recognize calls to alloca.  The man page 
for cc/xlc/c89 mentions that a flag is needed to do this, but I 
have not been able to find it in any of the documentation for cc
or alloca. 

Thanks in advance,
Todd