@agorrod: it compiles fine, but it fails to run:
```
dyld: lazy symbol binding failed: Symbol not found: _wiredtiger_config_parser_open
  Referenced from: ../../ext/datasources/helium/.libs/libwiredtiger_helium.so
  Expected in: flat namespace
dyld: Symbol not found: _wiredtiger_config_parser_open
  Referenced from: ../../ext/datasources/helium/.libs/libwiredtiger_helium.so
  Expected in: flat namespace
```

I thought it was something not declared in wiredtiger.in, but your change there looks correct to me.

I tested, and I can load/run wiredtiger_struct_unpack calls from the Helium code without any problem.

I've been hunting for awhile and I don't see anything wrong.  Any ideas how wiredtiger_config_parser_open is implemented differently from wiredtiger_struct_unpack?