[CMake] Make clean infelicity for Fortran submodules

[William Clodius]

CMAKE 3.12.0-rc3 claims to be able to handle Fortran submodules, and my limited testing of it suggest that it is doing an adequate job. One minor “feature” however is that make clean with the generated Makefile does not remove generated *.smod files. While their presence does not affect subsequent compilations, i.e., they are replaced if the corresponding *.mod files are absent, it would.d be nice if they were removed to reduce file storage.