cmake: attempt to work around the problem with VS 2019 ARM64 builds.

Author: guyharris

.appveyor.yml

@@ -26,6 +26,21 @@ environment:
   #
   # For now, we disable the remote capture build there.
   #
+  # To work around a rpoblem that has popped up in some places, wherein
+  # building for ARM64 gives a warning that _CountOneBits64 isn't
+  # defined, force the VS 2019 ARM64 builds to use the 10.0.22621.0
+  # SDK. This problem was happening to us, and some others online
+  # reported it, including
+  #
+  #    https://zhuanlan.zhihu.com/p/1935500669210572158
+  #
+  # which, after translation by Google Translate, indicates that the
+  # workaround is to use the older SDK.
+  #
+  # THe problem showed up after an upgrade of the AppVeyor image.
+  # It may be that the VS 2019 VC headers and the Windows 11 SDK
+  # (10.0.26100.0) don't work together.
+  #
   matrix:
       # VS 2017, WinPcap, 32-bit and 64-bit x86, with remote
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
@@ -93,12 +108,14 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       GENERATOR: "Visual Studio 16 2019"
       PLATFORM: ARM64
+      CMAKE_SYSTEM_VERSION: version=10.0.22621.0
       SDK: npcap-sdk
       REMOTE: -DENABLE_REMOTE=NO
       # VS 2019, Npcap, 64-bit ARM, with remote
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       GENERATOR: "Visual Studio 16 2019"
       PLATFORM: ARM64
+      CMAKE_SYSTEM_VERSION: version=10.0.22621.0
       SDK: npcap-sdk
       OPENSSL_ROOT_DIR: -DOPENSSL_ROOT_DIR=C:\OpenSSL-v33-Win64\bin
       # VS 2022, WinPcap, 32-bit and 64-bit x86, without remote