Catch unrecognised compiler directives in the compiler scanner.

Normally the preprocessor will catch these and report them as undefined
macros. But in case the compiler is run without the preprocessor, also
catch them in the compiler scanner. This will ensure the scanner properly
rejects directives that have additional garbage characters tacked on the
end.

lexor.lex

@@ -894,6 +894,11 @@ TU [munpf]
 	   << endl;
   }
 
+`[a-zA-Z_]+ {
+      yywarn(yylloc, "macro replacement not supported. "
+             "Use an external preprocessor.");
+  }
+
 
 `{W} { cerr << yylloc.text << ":" << yylloc.first_line << ": error: "
 	    << "Stray tic (`) here. Perhaps you put white space" << endl;