Cleanup handling of NETCDF_ENABLE_SET_LOG_LEVEL and NETCDF_ENABLE_SET_LOG_LEVEL_FUNC

[DennisHeimbigner]

The NETCDF_ENABLE_SET_LOG_LEVEL_FUNC option is apparently not used directly. But it is effectively used to set NETCDF_ENABLE_SET_LOG_LEVEL. This is not clear from the build files CMakeLists.txt and configure.ac. So this PR cleanups the code to make it more clear what is going on.

[WardF]

Looking at this, is there a reason not to just have NETCDF_ENABLE_LOGGING control the whole shebang? I can make that change, I'm just trying to figure out the usecase where we'd compile with logging but not want to enable nc_set_log_level.

[DennisHeimbigner]

I confess that the whole set of logging options is a bit opaque to me.
I would have agreed that having just a single option would make the most sense.
And no, I do not know of a use case for leaving set_level defined but do nothing.
So if you want to change it, I am ok with that, although it may take some unwinding.