Do not raise an error if os.environ does not contain PATH

Author: radarhere

setup.py

@@ -243,6 +243,8 @@ def _find_include_dir(self, dirname, include):
 
 
 def _cmd_exists(cmd):
+    if "PATH" not in os.environ:
+        return
     return any(
         os.access(os.path.join(path, cmd), os.X_OK)
         for path in os.environ["PATH"].split(os.pathsep)