You're right, C doesn't like empty assignments. But in this case that's probably wanted because configure is using the __GNUC__ defines as a way to check whether it's using GNU C at all, and which version. Any other compiler won't set those defines at all, and those that set it define it with the current version information.

As for the "fix". Debian uses the .deb format (short for "Debian"), and sometimes modifies the software so that it will run. Slackware, OTOH, uses (almost) exclusively vanilla (unmodified) code. But you can try and apply the Debian fix yourself, the patch is available here