Make the lexer distinguish between identifiers and defined macros.

This is just a minor style improvement for now. But the same
mechanism, (having the lexer peek into the table of defined macros),
will be essential when we add function-like macros in addition to the
current object-like macros.
diff --git a/glcpp.h b/glcpp.h
index 8472570..39d6d5d 100644
--- a/glcpp.h
+++ b/glcpp.h
@@ -52,6 +52,10 @@
 void
 glcpp_parser_destroy (glcpp_parser_t *parser);
 
+int
+glcpp_parser_macro_defined (glcpp_parser_t *parser,
+			    const char *identifier);
+
 /* Generated by glcpp-lex.l to glcpp-lex.c */
 
 int