Yes, you can place it there, or you can put it in a "slackbuild.options" file, along with other variables needed for making pkgs, then source that file with each SlackBuild script like this:
I'm currently in the same process of setting up my own build system, following the examples from various sources: LinuxPackgaes.net, Slackware, and LFS/BLFS (Linux From Scratch / Beyond Linux From Scratch)
I've elected to put the options files (including a separate one especially for KDE, called KDE.options) in the /etc directory, to be consistent with the Slackware setup of using /etc for configuration files
While I still have those variables in my startup script, I'm thinking of removing them as soon as I complete my package building system, so far I have KDE/QT fine-tuned to the point where I'm satisfied, but I still have my work cut out for me with the configure options for the rest of the Slackware source tree, including the extra/source/
directory...........There are still some old build scripts that do not follow the relatively recent buildscript naming convention of "package_name.SlackBuild", which I will be converting.................Also, I've created a "template.SlackBuild" file to help make the job easier, but it will still take some time to finish the project (and keep me out of trouble
).................The template will also help with any third-party apps that I eventually want to build and use....
If you need help with some specifics, let me know