The SlackBuilds.org templates allow you to set ARCH when you call the script:
Code:
ARCH=i686 ./gdal.SlackBuild
The newer Slackware scripts have similar blocks of code to handle ARCH. You will find more newer Slackware scripts in 14.1; to prefer the 14.0 scripts would seem perverse... anyway, to state what should be obvious, you can just run the other scripts through sed. Job done in less than a minute.
But that's not sufficient for a cross-compiling setup.
There are two ways of cross-compiling:
(1) Full cross-building, including configuring and linking. You will need to install a full set of Alien Bob's multilib packages on the x86_64 boxes and run his 32dev.sh script at the start of every cross-building session. Read all about it here:
http://www.slackware.com/~alien/multilib/
(2) Do the configuring and linking natively on i686, and use distcc to forward the compilation to your x86_64 hosts for speed. This is much, much easier. You don't need to set up a complete cross-toolchain. You don't even need to install Alien Bob's multilib -- not a lot of people know this, but Slackware's standard x86_64 gcc packages can cross-compile i486/i686. To do it this way, see my sig.