Since commit v3.5.0-rc1~241^2~1 (cmFortranParser: Parse #line
directives, 2015-11-02) our Fortran dependency scanner parses `#line`
directives to extract the named files.  However, some compilers produce
`#line` directives that name directories instead of files.  Work around
such cases by verifying that the extracted path names a file and not a
directory.