HEADS UP: building RELENG_5/6 jails on RELENG_7/HEAD

[Ade Lovett]

Hi folks,

Just in case y'all don't follow cvs-src, fjoe@ recently committed a  
fix to RELENG_6 to allow it to be built on a RELENG_7 or HEAD system.   
I requested a further MFC of this fix back to RELENG_5 which he  
graciously did.

This negates the need for the preJailBuild hook hack with the  
binutils.diff that most of us have lying around.

-aDe