make: fix GCC warnings

*** 199,209 ****
                          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++);
                                  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,
--- 199,209 ----
                          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 != 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,