Logic to install plugins for cmake netcdf-c builds

[vanderwb]

While plugin installation support is provided for the autotools build method, this was apparently not implemented for CMake builds of netcdf-c. This PR aims to rectify this inconsistency. As with many configurations, the process for turning plugin install on changed in 4.9.3.

From 4.9.0 to 4.9.2, setting PLUGIN_INSTALL_DIR to the desired directory is all that is required (assuming we are building a shared library).

Meanwhile, in 4.9.3, a new variable is introduced and documented which serves the same purpose - NETCDF_WITH_PLUGIN_DIR. However, setting this doesn't seem to be sufficient as there is no logic in the top-level CMakeLists.txt to set ENABLE_PLUGIN_INSTALL, which is required for plugin libraries to be installed in the plugins subdirectory. This seems like a bug, but we can work around it by setting this manually.

[vanderwb]

@WardF and/or @skosukhin - just sending you a ping on this since I submitted it over the holidays. The issue I'm attempting to fix here has lead to problems for downstream packages: we initially saw it with nccmp as it produces a "filters not found" error when using CMake builds of netcdf-c in its current form.