Update: Okay, nothing is overwritten (whew!), but /usr/local/opt still 
wasn't used and /usr/local/bin/xemacs is a soft link to the executable 
name 'xemacs-21.5-b34'.  How is configure supposed to work--it's not 
doing what --help says? For now I created aliases to point to the 
desired xemacs and I'm back to normal (except for my ~/.xemacs dir which 
isn't liked by xemacs beta).

I tried to install xemacs beta alogside xemacs stable so I tried to 
configure it so:  ./configure --prefix=/usr/local/opt  but it installed 
in /usr/local and overwrote my xemacs stable: AARGH!!  The generated 
configure.log has the correct prefix but the generated Makefile does NOT 
(discovered after the fact of the erroneous install).