adapter/parport: Deprecate direct I/O support

We deprecate direct I/O support in favor of ppdev for the following
reasons:

  - Linux supports ppdev since ~2.4 (released ~24 years ago) and it is
    enabled by default on major distros (Ubuntu, Fedora). So it is
    effectively ubiquitous
  - FreeBSD provides no direct I/O support, so ppdev (ppi) is the only
    viable option
  - Direct I/O requires root/elevated privileges which is inadvisable
  - Removing direct I/O reduces build and driver complexity and yields
    a smaller, easier-to-maintain codebase
  - Supporting only ppdev allows us to simplify the codebase by using
    device files (e.g., /dev/parport0) instead of numeric identifiers

Windows is the only rationale to keep direct I/O, but the user base
appears minimal to nonexistent and no active contributors can test the
Windows driver.

Change-Id: Ia6d5ed6e8c5faa2a9b4919ca97c5cf9033372a64
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/9151
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

configure.ac

@@ -356,7 +356,7 @@ AC_ARG_ENABLE([parport_ppdev],
 
 AC_ARG_ENABLE([parport_giveio],
     AS_HELP_STRING([--enable-parport-giveio],
-      [Enable use of giveio for parport (for CygWin only)]),
+      [Enable use of giveio for parport (deprecated, for CygWin only)]),
     [parport_use_giveio=$enableval], [parport_use_giveio=])
 
 AC_ARG_ENABLE([gw16012],
@@ -825,6 +825,12 @@ AS_IF([test "x$build_gw16012" = "xyes"], [
   AC_MSG_WARN([Gateworks GW16012 JTAG adapter is deprecated and support will be removed in the next release!])
 ])
 
+AS_IF([test "x$parport_use_giveio" = "xyes" || test [ "x$enable_parport" != "xno" -a "x$parport_use_ppdev" = "xno"]], [
+  echo
+  echo
+  AC_MSG_WARN([Parallel port access with direct I/O is deprecated and support will be removed in the next release!])
+])
+
 echo
 echo
 echo OpenOCD configuration summary