Do not attempt to build extensions on platforms other than CPython.
diff --git a/setup.py b/setup.py
index 3e34cfe..fbc6795 100644
--- a/setup.py
+++ b/setup.py
@@ -122,6 +122,8 @@
         return False
 
     def ext_status(self, ext):
+        if 'Java' in sys.version or 'IronPython' in sys.version or 'PyPy' in sys.version:
+            return False
         if isinstance(ext, Extension):
             with_ext = getattr(self, ext.attr_name)
             return with_ext