The only header a user needs is dll.hpp, unfortunately it is not properly installed by the source tree. On top of this rar2fs add some built-in extensions to libunrar.so and thus requires a few more non-standard headers. An option would be to ship the required headers with rar2fs but that has the unwanted side-effect that new versions of unrar source would require a new rar2fs release. There might also be licensing issues involved.