cctk_Loop.h used constructs like
#define FOO(x) cctk_loop_/**/x
to concatenate identifier names. This only works when using
cpp --traditional but not for an ANSI C preprocessor that requires
#define FOO(x) cctk_loop ## x
Cactus’s old header files in
cctk_Types.h support both options via an ifdef.
All current option lists use
cpp --traditional for
FPP and Cactus’s
linux built in architecture file does the same, and
cctk_Loop.h is (as of 2020-04-03) broken for other reasons (see http://lists.einsteintoolkit.org/pipermail/users/2020-February/007298.html) so this issue has not yet show up.