Commit 8918a9cf introduced macro
`#define rb_cData rb_cData()`.  This deleting `VALUE rb_cData;`
declaration was then macro-expanded into `VALUE rb_cData();`.  This
worked by accident because the expanded expression happen to be a K&R
style function declaration.

This is rather complicated and I guess unintended.  Just delete the line
to keep things simple straight forward.