2011-01-09.

Gentoo system compiled around 2009-09, with very little updating since.

Want to keep the normal 64-bit mplayer that came with gentoo (this will
be used for almost all videos/audio, and for recoding), but also want 
one that can use various obscure proprietary codecs such as sipr.so 
for certain old realaudio.  Gentoo no longer stocks a binary 32bit 
version.  

So: an incantation is needed to cause 

ls  /usr/portage/app-emulation/

emerge -s emul-linux-x86-soundlibs emul-linux-x86-xlibs emul-linux-x86-medialibs emul-linux-x86-baselibs emul-linux-x86-compat emul-linux-x86-cpplibs

Then, with assistance from http://ubuntuforums.org/showthread.php?t=739011, 
the following line did a suitable configure:

$	CC="gcc -m32"  ./configure  --disable-runtime-cpudetection --target=i686-linux --libdir=/usr/lib32 --prefix=/opt/mplayer32 --codecsdir=/usr/lib32/codecs --enable-x11 --enable-xv --enable-alsa --enable-ossaudio --disable-gui

(
The prefix was just my preference, for keeping this program entirely 
in its own little compartment.  The codecsdir is whereever the big 
bundle of 32bit binary codecs (once called win32codecs) has been put.
This was also tried initially with  
	--enable-static
to make a more easily moved binary, but there were clearly some 
libraries (Xv.a was the first error from file not found on 32bit path) 
that were only present in dynamic form (libXv.so); so configure and 
make were repeated without the static option.
)

Then, 
$	make
(just normal 64-bit make, as the configure script has already set 
things up with suitable options for the 'cross'-compilation).

Finally, of course, the installation:
#	mkdir /opt/mplayer32 ; chown myusername /opt/mplayer32
$	make install
#	chown -R root:root /opt/mplayer32
#	cd /opt/bin && ln -s ../mplayer32/bin/mplayer mplayer32

And it worked fine at once.
Annoying though it is to waste any time over silly codecs and 
proprietary software, it is satisfying that this was so easy, 
thanks largely to the above link having done my work for me in 
finding the needed options.