make: fix GCC warnings

@@ -306,12 +306,12 @@
 extern  Boolean         stdout_stderr_same;
 extern  Dependency      suffixes;
 extern  Name            suffixes_name;
 extern  Name            sunpro_dependencies;
 extern  Boolean         target_variants;
-extern  char            *tmpdir;
-extern  char            *temp_file_directory;
+extern  const char      *tmpdir;
+extern  const char      *temp_file_directory;
 extern  Name            temp_file_name;
 extern  short           temp_file_number;
 extern  wchar_t         *top_level_target;
 extern  Boolean         touch;
 extern  Boolean         trace_reader;