Avoid printing a space at the beginning of lines in the output.

This fixes more differences compared to "gcc -E" so removes several
cases of erroneously failing test cases. The implementation isn't very
elegant, but it is functional.
diff --git a/glcpp.h b/glcpp.h
index 5432a31..c25e29c 100644
--- a/glcpp.h
+++ b/glcpp.h
@@ -101,6 +101,7 @@
 	yyscan_t scanner;
 	struct hash_table *defines;
 	expansion_node_t *expansions;
+	int just_printed_separator;
 };
 
 void