I suggest to drop explicit support for Fortran 77 in Cactus. Fortran 77 is, for all practical purposes, a subset of Fortran 90, and thus Fortran 77 code can be compiled by Fortran 90 compilers.
There is currently no platform that has a Fortran 77 and no Fortran 90 compiler, and there is no Fortran source code in Cactus that cannot be compiled by a Fortran 90 compiler.
In a way, supporting Fortran 77 as language is similar to supporting K&R C as a language. We don't do this either.
I suggest to remove/ignore all configuration options regarding Fortran 77, and to compile .f77 and .F77 files with a Fortran 90 compiler. This change will simplify the configuration stage of Cactus. I don't expect any user to notice.