gdal 3.9.0, C++17, charconv, gcc7

[Greg Troxel <gdt%lexort.com@localhost>]

gdal 3.9.0, now in pkgsrc, is documented by upstream to need c++17.
Except it was hard to find and I didn't notice.  mef@ changed that
(USE_CXX_FEATURES).  So far this is unremarkable.

I find that:

  - c++17 is coded to require gcc7, but gcc says it needs 8.  Our c++17
    code in compiler/gcc.mk doesn't mention that gcc7 does not really
    support c++17, or that one also needs to set specific features.

  - gdal uses charconv.  That's part of c++17, so that's legit, but 7
    does not have it

  - charconv as a feature is coded to require 8


I realize there are a lot of considerations, but if we document that
c++17 as a CXX_FEATURE should fully support the language, don't we need
to make it 8?

Would that leave charconv as "uses charconv, but in general does not
need c++17"?

Or something eles?