It is declared in cctk_Termination.h, but this file is included from cctk.h. Users should only include cctk.h.
We should add a Fortran wrapper for this function.
Are you sure about this? I just tried adding CCTK_TerminateNext(n); to a C source file (EinsteinInitialData/Hydro_InitExcision/src/Hydro_InitExcision.c from trunk) and it fails to compile with "warning: implicit declaration of function ‘CCTK_TerminateNext’". Doing a 'grep cctk_Termination.h' in src/include only finds it in the file cctk_Termination.h.
Since this is just a warning the code compiles (and links eventually) unless one make missing prototypes an actual error (or compiles user -std=c99), but there is no checking of parameters (eg. one could call it without passing cctkGH which would likely crash the code).
Reading your explanation I think my bug report should be changed to say that cctk.h fails to inlucde cctk_Termination.h then