Hi there, I casually (well not so properly 
) built today the 6.0.3 version on Debian 12, I suspect the same issue applies to any Linux with a modern GCC (i.e. 10+). Polsar Makefiles needs to consider changes in the GCC toolchain that now assumes -fno-common or fix sources to use extern declaration when due. An easy fix is adding -fcommon to every CFLAGS arg, which is what I did a few minutes ago to have a working build.
/me with my Debian Developer hat on.
Eventually consider to send me a note for clarification about patching, just in case.
-cheers
PS: I created a mirror of the sources for Linux on Github here including a patch to allow building on a current Linux system.