Add `enable-legacy-macros` option.

[WardF]

As observed in #3017 and #3018, the change introduced in #2911 (discussed in #2858), re: refactoring the macro _FillValue to NC_FillValue has caused some issues for projects; it is easily fixed, but it is a problem when the issue potentially requires changing a projects code. While it is tempting to say it is the problem of downstream projects, we should provide a mechanism to ameliorate the inconvenience and also recognize that for larger projects, there are often processes in place which make 'just change your code' more difficult than it sounds.

I'll add an option, 'enable-unsafe-macros', which will be off by default but, when true, will define _FillValue in parallel with NC_FillValue.

[edwardhartnett]

I don't think this should be an option, it should be something that always happens.

_FillValue is part of the API, it's been used for decades. Provide NC_FillValue but also leave _FillValue defined. If end-users have a problem with it, they can undef it in their netcdf.h file.

Removing _FillValue will just cause everything to break. It's a direct violation of our commitment to backward compatibility of code. Whatever harm is done by leaving _FillValue in, pales with the harm of breaking everyone's code on a netcdf-c update. (If you want people to stop updating netcdf-c and just stay with 4.9.2 for the next 20 years, this is a good way to achieve it.)

If you search through Russ' old presentations you will find the "Certificate of Backwards Compatibility" he used to display, in which he called the backward compatibility "sacrosanct". Users depended on that guarantee and followed documented best practices using the _FillValue macro.

If you must have an option, then make it default to including _FillValue, because that's what everyone needs to do.