Add automatic TMPDIR setup/teardown to GNU Make

We now guarantee TMPDIR will be defined on a per build rule basis. It'll
be an absolute path. It'll be secure and unique. It'll be rm -rf'd after
the last shell script line in your build rule is executed. If $TMPDIR is
already defined, then it'll be created as a subdirectory of your $TMPDIR
and then replace the variable with the new definition. The Landlock Make
repository will be updated with examples shortly after this change which
shall be known as Landlock Make 1.1.1.

See #530

third_party/make/job.h

@@ -41,6 +41,7 @@ struct child
 
     struct file *file;          /* File being remade.  */
 
+    char *tmpdir;               /* Temporary directory */
     char *sh_batch_file;        /* Script file for shell commands */
     char **command_lines;       /* Array of variable-expanded cmd lines.  */
     char *command_ptr;          /* Ptr into command_lines[command_line].  */