Cactus passes Fortran code (at least with extensions .F77 .F and .F90) through the C preprocessor in FPP passing FPPFLAGS.
This cpp must use traditional (pre-ANSI) behaviour, ie be string and not token based to preserve whitespace in Fortran fixed format code and handle concatenation using
Getting this wrong by setting eg only FPP in an option list results in strange error messages.
Cactus' configure should check that FPP and FPPFLAGS behave as expected. Eg by processing:
without, so one can
grep for the expected string.