introduce a couple of new turn-off-gcc-warning variables and use them.

GCC 14 has a new annoying calloc() checker that we turn off in a bunch
of places, and there are a few more dangling-pointer issuse that come up,
but seem bogus.

Add nbperf(1), a minimal perfect hash function generator.
Implemented are the 3-graph BDZ algorithm as well as the
2-graph and 3-graph CHM algorithms. All algorithms have expected
linear run time and the smallest functions need around 2.85 bit/key.