Add legacy macro option

[WardF]

• Fixes Add enable-legacy-macros option.  #3029

Adds legacy macro option to enable _FillValue, any other macros which may reveal themselves. As discussed in #2858, the issue with _FillValue is that it can, in some circumstances, conflicts with a symbol libc++18. While _FillValue is historically important and widely used, (and technically tricky to move away from), we can't avoid the fact that _ is a reserved identifier and will potentially cause problems for users as libc++18 and later become more common in the wild.

We replaced _FillValue with NC_FillValue in #2858, and we encourage users to adjust their software to account for this. But not all of our users are maintaining software they rely upon, and we do not (and cannot) put people in a position where software they rely upon but are not equipped to modify will not compile.

Invoking --enable-legacy-macros (autotools) or -DNETCDF_ENABLE_LEGACY_MACROS=TRUE (cmake) will define the _FillValue macro, for use when building netCDF as part of a larger, un-modified technical ecosystem.

[edwardhartnett]

I believe this should have the opposite default setting, as the number of users who will encounter problems with _FillValue is tiny compared to the number that will have problems with its absence.

At least give users a few releases with the new macros available but not yet mandatory.

[WardF]

@edwardhartnett It would be great if we could get more release candidate exposure so that we had a better feel for this. Flipping it (as you suggest) doesn't do any harm and could (as pointed out) be better in the short run. I have documentation to add before I was going to merge this, so I'll flip the polarity for the next rc. No harm in changing a considered opinion, after all ;).

[gsjaardema]

For some customer input, I am fine with either option. We have already added code to test whether NC_FillValue is defined and if it isn't, then use the old _FillValue

[czender]

Echoing @gsjaardema , I just amended NCO to use NC_FillValue, which NCO #defines if/when netCDF does not.