I updated libdrm, mesa and xf86-video-ati from git by following instructions from Radeon KMS thread. Everything works fine, except now I would like to create 32-bit packages for those. As per Alien's multilib 32-bit compiling instructions, I left arch as x86_64, changed LIBDIRSUFFIX to "" (from "64") and run /etc/profile.d/32dev.sh before running SlackBuild script. Compilation went just fine (except I had to install/convert 32-bit udev package).

However, one thing is bothering me: both packages, 32-bit and 64-bit have same name. I checked contents of both and they contain /usr/include. Obviously if I try to install 32-bit it will overwrite /usr/include from 64-bit. Right?

I'm curious how to override this problem or I'm missing something here?

The files installed in /usr/include are not (generally) ABI specific so it doesn't matter if they're overwritten. There are a few packages that I've had to create a stub header for. I don't remember if that was to separate the x86_64 ABI's or the different architectures (i.e., x86_64 from MIPS).

Libraries are ABI specific which is the reason x86_64 machines are supposed to have a lib and lib64 directory. There are some files, like glib-config, that contain ABI specific information. Theses files are used by other packages instead of metadata (*.pc) files.