Disable szip by default, because it's non-free, following the general
pkgsrc notion that free software should not depend on non-free
software (because that results in not having distributable binary
packages, and makes the build fail) unless skipping the dependency
renders the software non-useful.

(The hdf web page says that the default binary builds of hdf include
szip support, but then go on about how users that don't qualify for
the no-cost license have to install a different version.)

If someone who grasps hdf5 can say that hdf5 w/o szip is generally not
useful, we can turn this back on and disable by default the hdf5
option in octave.

(gdt)