make: fix GCC warnings

@@ -199,11 +199,11 @@
                         save_makefile_type = makefile_type;
                         makefile_type = reading_nothing;
                         if (doname(makefile_name, true, false) == build_dont_know) {
                                 /* Try normalized filename */
                                 string_start=get_wstring(makefile_name->string_mb);
-                                for (string_end=string_start+1; *string_end != NULL; string_end++);
+                                for (string_end=string_start+1; *string_end != L'\0'; string_end++);
                                 normalized_makefile_name=normalize_name(string_start, string_end - string_start);
                                 if ((strcmp(makefile_name->string_mb, normalized_makefile_name->string_mb) == 0) || 
                                         (doname(normalized_makefile_name, true, false) == build_dont_know)) {
                                         n = access_vroot(makefile_name->string_mb,
                                                  4,