Without the change build on toolchain with `-fno-common` default fails as:
$ nix build --impure --expr 'with import ./. {}; cdrkit.override { stdenv = clang12Stdenv; }' -L
...
cdrkit> ld: CMakeFiles/genisoimage.dir/apple.o:(.bss+0x2040):
multiple definition of `outfile'; CMakeFiles/genisoimage.dir/genisoimage.o:(.bss+0x280): first defined here
