basically it's the same Slackbuild script used in Slackware (32 bit), with minor modifications, so it should be working as well in 64 bit machines. I don't know why your system prefer to use clang instead of the default GCC

I have updated the SlackBuild to build 1.7.0 now and also cups-filters to 1.0.40
for those who have been using 1.6.x SlackBuild, you only need to change the VERSION line and it should work

One good news is that newer cups-filter (1.0.40) no longer depends on avahi, so for someone to build CUPS 1.6+, they would only need this packages as a minimum requirements
- cups-filters
- qpdf

This means removing avahi and libdaemon as requirements, meaning less packages need to be added in future Slackware releases

Good news. Now I will remove the packages. Version 1.7.0 fixes also the compiling bug on 64bit machines: no need to force gcc compiler anymore, the slackbuild works as is.
BTW, I also upgraded Gutenprint to the current CVS version (pre 5.2.10): it should better handle the Canon Pixma printer I own. However, building fails generating documentation, and excluding doc in configure seems to have no effect. My workaround was to replace the /doc tree in the cvs source with the one from 5.2.9 release (documentation seems just the same).