While contrib/ipv6calc.spec is generated from contrib/ipv6calc.spec.in, the pregenerated file is shipped in the source tree and resulting tarball. This is unlike e.g. Makefiles, that are also generated from Makefile.in, but in this case no pregenerated Makefiles being shipped.
This results into a dirty source tree when running "make; make clean" and requires workarounds in the Debian package.