It is in include/petsccusp.h but has includes from the PETSc source code tree (which won't even work for --prefix installs).
Likely this need to be refactored or discarded or something
#if !defined(__PETSCCUSP_H)
#define __PETSCCUSP_H
/*
This should only be included in user code that uses CUSP directly and hence
the file name ends with .cu
*/
#include <../src/vec/vec/impls/dvecimpl.h>
#include <../src/vec/vec/impls/seq/seqcusp/cuspvecimpl.h>
#endif
