Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Work around build breakages with GCC 4.2.

Reported by: tinderbox

Reduce diff with upstream.

Reimplement support for the ** (exponent) gnu extension, make it available thought the -g (mimic gnu) option

Reviewed by: cognet
Approved by: cognet
Discussed with: espie@OpenBSD.org (upstream)