The Cactus build system currently cds into the configuration's scratch directory before compiling any file, and accumulates all Fortran module (.mod) files there. This results in a very large directory, and does not separate thorns from each other. This also makes it difficult to delete all built files from a single thorn, which is often convenient for debugging.
Instead, Fortran module files should be written into the thorn's object directory. Cactus should then set the Fortran include path correctly so that other thorns' module files are found, if this thorn declared the dependency onto the other thorns.