configure に --with-opt-dir=/usr/local と指定すると、
  configure: WARNING: unrecognized options: --with-opt-dir
という警告が出ます。

% autoconf
% ./configure --with-opt-dir=/usr/local
configure: WARNING: unrecognized options: --with-opt-dir
checking build system type... i686-pc-linux-gnu
checking host system type... i686-pc-linux-gnu
checking target system type... i686-pc-linux-gnu
checking for gcc... gcc
...
checking for if make is GNU make... yes
checking for nroff... /usr/bin/nroff
.ext/include/i686-linux/ruby/config.h updated
ruby library version = 1.9.1
configure: creating ./config.status
config.status: creating Makefile
configure: WARNING: unrecognized options: --with-opt-dir
% uname -a                              
Linux nute 2.6.26-1-486 #1 Fri Mar 13 17:25:45 UTC 2009 i686 GNU/Linux
-- 
[田中 哲][たなか あきら][Tanaka Akira]