cause plugin path specified in configure step to be automatically added to the netcdf plugin path

[edwardhartnett]

With the very helpful #3024 , we can now more easily approach the problem of what to do with the plugin path that the users specified in the configure step.

On library initialization, that path should be added to the plugin path.

This will cause everything to work out of the box for all users. Advanced users, thanks to #3024, will easily have the ability to take charge of the plugin path, and get full control. All other users will use the plugin path specified at configure time, and will not even have to know about it.

On HPC systems, sysadmins can build and install netCDF, and set whatever plugin directory they want, and users will get everything working out of the box without an environment variable, or even knowing anything about plugins or plugin paths.

This will reduce support requests to netCDF, because a plugin path problem will be perceived as a netCDF bug. Making it work out of the box without user involvement allows scientists to get right at their data without messing around with settings.

[DennisHeimbigner]

Ed, you don't get off the hook that easily :-)
We also need to take into account the mechanisms of HDF5 and what is already in place.
Specifically, we have the following info to deal with:

1. HDF5_PLUGIN_PATH at build time
2. HDF5_PLUGIN_PATH at run time
3. --with-plugin-dir option

There is probably little use of No. 4, but there is almost certainly use of No. 3.

[edwardhartnett]

HDF5_PLUGIN_PATH plays no role at build time. We should not care what it is at build time.

At run time, HDF5_PLUGIN_PATH is used by HDF5, not by netCDF. I would suggest we do nothing with HDF5_PLUGIN_PATH.