Log Message:
On Darwin, add gsed to USE_TOOLS and patch the icestorm Makefile to
stop working around a behavioral difference in the Darwin sed(1) -i
option relative to other sed implementations.

If textproc/nbsed supported the -i option, the USE_TOOLS change would
not be necessary.  But since the Darwin system sed(1) is not used in
pkgsrc at the moment, the Makefile patch would still be required.

See also PR pkg/57970.