This is mostly for XDG/4 compatibility.

Scintilla v5.5.7 officially requires at least GCC v9, but if it's
only the charconv header that's required from newer releases, v8.1
will probably do as well. We assume so until proven wrong.

* It is even more robust now and does not remove the large document
  optimization.
* And actually while problems were caused on NetBSD 10 on ARMv6,
  this likely wasn't a problem on all NetBSD ports and wasn't
  restricted to NetBSD. At the very least Haiku 32-bit was affected
  as well.

It should be more robust now.
The changes on CellBuffer should be reviewed again, though.

* We can use the new SCI_SCROLLVERTICAL instead of SCI_SETFIRSTVISIBLELINE.
* The upgrade will also ease submitting the NetBSD-support patch.

* They have a newer version in pkgsrc, but it's not even available
  as a binary package on the arm6.
* Has some glitches, e.g. does accept the ASCII 27 in tutorial.ms,
  but it's probably not worth to work around.

* It requires a forced refresh on startup (even though that should be the
  default). Otherwise, it wouldn't print the info line correctly.
* Redirect stdin and pass it to newterm() to fix key queuing.
  Probably necessary for supporting ncurses on NetBSD as well.
* Avoid doupdate() if screen is too small: fixes crashes for very
  small windows.
* Updated Scintilla: There were some implicit typing assumptions,
  that are broken by this platform.

If we find libncursesw or libncurses, but without pkg-config and if
ncurses does not install its curses.h into the standard search path,
we might theoretically pick up another compatible Curses' curses.h.
Better guard against this.

* pkg-config check for `ncurses` fails if it failed previously for `ncursesw`.
  This is the case e.g. for ncurses from NetBSD's pkgsrc.
* No longer assume that any libncurses is not enhanced (X/Open compatible).
* SciTECO and Scinterm require to find a curses.h in the include paths.
  The ncurses check must therefore not be limited to the first best
  ncurses/ncurses.h and the like.
* We now always check for X/Open compatibility and always require
  a curses.h in the standard directories or as given by pkg-config.
* AX_WITH_CURSES was radically rewritten and is now called AX_WITH_NCURSES.
* --with-interface=netbsd-curses gets its own detection code.
  It always requires a curses.h in the standard paths and a libcurses.
  It should now be fixed for real NetBSD installations if the ncurses
  port is installed as well.
* Unified all of the curses-arguments to CURSES_CFLAGS and CURSES_LIBS.
  There is no reason we need PDCURSES_CFLAGS, XCURSES_CFLAGS etc.