plugins should be installed by default when zstd is found by autotools, like the CMake build does

[edwardhartnett]

We look for zstd and if we find it, we turn on zstd.

But then we don't install plugins unless the user explicitly tells us to. So zstd will be broken.

What would make more sense would be to install the plugins by default, if zstd is found, in the default location. This will cause zstd to automatically work for users who don't want to worry about HDF5_PLUGIN_PATH.

Users that do want to care about the plugin path can still install plugins where they like, and use the HDF5_PLUGIN_PATH to specify their location. But if the user doesn't say, let's just install plugins in the default location.

The CMake build actually does the correct thing, defaulting to install the plugins in the default plugin directory.