So, hypothetically, suppose cusp had a library too, but still no installer. Then the directions would say to put the headers in /usr/local/cuda/include and the library in /usr/local/cuda/lib, right? Would that change the definition of a prefix? What if it was a manual page? I don't see how lack of an automatic install changes the meaning of a prefix. Any reasonable person would be outraged if a header-only package had an install target and installed to $prefix/*.h instead of $prefix/include/*.h.