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

[Greg Troxel]

also, I am far from a cmake expert, but I looked in the CMakeFiles  and
cannot find where they

  - try to run the compiler with --std=c++17
  - fail the build if that does not work with a message that makes it
    clear that the compiler doesn't work --std=c++17
  - if it works, add --std=c++17 to the compiler invocations, because
    requiring a c++17 compiler is fine, but one can't assume that it
    uses it by default

Maybe it does all that and my cmake-fu is too weak.