proper fix for C++17?

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

kicad was recently updated (yay) but it now depends on C++17.  In theory
gcc7, in NetBSD 9, supports C++17, but it in fact does not, failing to
provide <charconv>.

With the following, it builds and runs.   I picked gcc10 because
  - that's what is in NetBSD 10
  - it was installed already because something else needs it

Do people think this is ok, and if not do we have documentation
somewhere about which versions are necessary for which C++ languages?