cmFortranParser: Skip #line directives that do not name files

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.

Source/cmFortranParserImpl.cxx

@@ -229,7 +229,7 @@ void cmFortranParser_RuleLineDirective(cmFortranParser* parser,
   cmSystemTools::ConvertToUnixSlashes(included);
 
   // Save the named file as included in the source.
-  if (cmSystemTools::FileExists(included))
+  if (cmSystemTools::FileExists(included, true))
     {
     parser->Info.Includes.insert(included);
     }