Remove unused private instance variable

btag now compiles again in OS X without warnings.
No need to set up CPack anymore

We don't publish tarballs anymore, GitHub doesn't allow it. Makes
distribution easier, too.
No need to set up debug flags

Leave it up to CMake to figure out the right flags.
Get rid of -Werror

While warnings are undesirable, -Werror might break the build even
on situations that are 100% harmless for the end user. In fact, it
just did that with unused private instance variables and recent
updates to the OS X compiler.