[gdal-dev] doc bugs about prereqs, surfaced by updating pkgsrc for 3.9

[Greg Troxel]

Even Rouault <even.rouault at spatialys.com> writes:

> [cmake]

Thanks for cluing me in; that  all makes sense.

> No idea if you need gcc 7 or 8. I don't think we use super advanced
> C++17 features, so gcc 7 might be fine, but only experimentation would
> confirm that. I believe the minimum we test on our CI is gcc 9.4 that
> ships with Ubuntu 20.04

7 fails, because gdal uses <charconv> and that only appeared in 8.  10
works, but you know that.  I am trying with 8, but gcc8 is still
building.  I'll report back if that fails.