Fortran: Warn when dependency scanning fails to parse a source file

We expect to handle all relevant statements and ignore those that we do
not understand.  Warn if this process ever fails.  Otherwise dependency
information may be silently left out.

Source/cmDependsFortran.cxx

@@ -130,6 +130,12 @@ bool cmDependsFortran::WriteDependencies(const std::set<std::string>& sources,
     if (cmFortran_yyparse(parser.Scanner) != 0) {
       // Failed to parse the file.  Report failure to write dependencies.
       okay = false;
+      /* clang-format off */
+      std::cerr <<
+        "warning: failed to parse dependencies from Fortran source "
+        "'" << src << "': " << parser.Error << std::endl
+        ;
+      /* clang-format on */
     }
   }
   return okay;

Source/cmFortranParser.h

@@ -146,6 +146,9 @@ struct cmFortranParser_s
   // Buffer for string literals.
   std::string TokenString;
 
+  // Error message text if a parser error occurs.
+  std::string Error;
+
   // Flag for whether lexer is reading from inside an interface.
   bool InInterface;
 

Source/cmFortranParserImpl.cxx

@@ -164,11 +164,9 @@ int cmFortranParser_GetOldStartcond(cmFortranParser* parser)
   return parser->OldStartcond;
 }
 
-void cmFortranParser_Error(cmFortranParser* /*unused*/, const char* /*unused*/)
+void cmFortranParser_Error(cmFortranParser* parser, const char* msg)
 {
-  // If there is a parser error just ignore it.  The source will not
-  // compile and the user will edit it.  Then dependencies will have
-  // to be regenerated anyway.
+  parser->Error = msg ? msg : "unknown error";
 }
 
 void cmFortranParser_RuleUse(cmFortranParser* parser, const char* name)