ignore-value: prefer GCC version back through 2.0

The code didn't match the comments, so I did a bit of software
archaeology.  GCC 2.0 seems to support __extension__ and
__typeof__, so fix both code and comments to use 2.0.
* lib/ignore-value.h (ignore_value): Use __extension__ and
__typeof__ for GCC 2.0 through 3.3, too.