make sublime build work on Windows

Sublime Text/Oberon/Oberon.sublime-build

@@ -1,5 +1,7 @@
 {
     "cmd": ["build.py", "--file=$file", "compile"],
     "file_regex": "^[ ]*File \"(...*?)\", line ([0-9]*)",
-    "selector": "source.oberon"
+    "selector": "source.oberon",
+    "working_dir": "${project_path:${folder}}",
+    "shell": true
 }