windows: Fix absolute-header.m4 for newer msys2 appearing as cygwin.

MSYS2 is trying to move closer to cygwin and as part of that effort
they changed reporting of the system triplets from msys to cygwin:
  https://www.msys2.org/news/#2025-06-20-replacing-x86_64-pc-msys-with-x86_64-pc-cygwin

This broke our builds in AppVeyor, since detection of absolute paths
for system headers is now using the wrong path separator list:

 checking absolute name of <stdio.h>... ""
 checking absolute name of <string.h>... ""

Fix by adding cygwin to the list for Windows-style paths.  With that,
we can properly detect the headers again:

 checking absolute name of <stdio.h>...
 "C:\\Program Files (x86)\\Windows Kits\\10\\include\\10.0.22000.0\\ucrt\\stdio.h"
 checking absolute name of <string.h>...
 "C:\\Program Files (x86)\\Windows Kits\\10\\include\\10.0.22000.0\\ucrt\\string.h"

We used the 'Previous' variant of the image for AppVeyor before to
avoid this issue, but now even the previous image has the new MSYS2
and CI is fully broken without the fix.

Acked-by: Mike Pattrick <[email protected]>
Acked-by: Kevin Traynor <[email protected]>
Acked-by: Eelco Chaudron <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>

Author: igsilya

appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.0.{build}
-image: Previous Visual Studio 2019
+image: Visual Studio 2019
 branches:
   only:
   - main

m4/absolute-header.m4

@@ -68,7 +68,7 @@ AC_DEFUN([gl_ABSOLUTE_HEADER_ONE],
   esac
 changequote(,)
   case "$host_os" in
-    (mingw*|msys*)
+    (mingw*|msys*|cygwin*)
       dnl For the sake of native Windows compilers (excluding gcc),
       dnl treat backslash as a directory separator, like /.
       dnl Actually, these compilers use a double-backslash as