make: fix GCC warnings

@@ -187,12 +187,12 @@
         Boolean         stdout_stderr_same;
         Dependency      suffixes;
         Name            suffixes_name;
         Name            sunpro_dependencies;
         Boolean         target_variants;
-        char            *tmpdir = NOCATGETS("/tmp");
-        char            *temp_file_directory = NOCATGETS(".");
+        const char      *tmpdir = NOCATGETS("/tmp");
+        const char      *temp_file_directory = NOCATGETS(".");
         Name            temp_file_name;
         short           temp_file_number;
         time_t          timing_start;
         wchar_t         *top_level_target;
         Boolean         touch;                          /* `-t' */