Configure changes for HAS_NON_INT_BITFIELDS

Configure changes for HAS_NON_INT_BITFIELDS

This probe has been available without my realizing it had been merged.
Some compilers, chiefly IBM, use only integer-size bitfields, warning
when presented with other-sized ones.  Allowing other-sized fields is an
extension to the C standard.

These warnings don't affect the correctness of the code generated, but
very many are generated per run, potentially overwhelming the reader
into not noticing warnings that are important.