Try add_compile_definitions(), step3

Author: fxlb

.appveyor.yml

@@ -95,14 +95,14 @@ environment:
       PLATFORM: ARM64
       SDK: npcap-sdk
       REMOTE: -DENABLE_REMOTE=NO
-      POPULATIONCOUNT64: -DPopulationCount64=1
+      POPULATIONCOUNT64: -DPOPULATIONCOUNT64=1
       # VS 2019, Npcap, 64-bit ARM, with remote
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       GENERATOR: "Visual Studio 16 2019"
       PLATFORM: ARM64
       SDK: npcap-sdk
       OPENSSL_ROOT_DIR: -DOPENSSL_ROOT_DIR=C:\OpenSSL-v33-Win64\bin
-      POPULATIONCOUNT64: -DPopulationCount64=1
+      POPULATIONCOUNT64: -DPOPULATIONCOUNT64=1
       # VS 2022, WinPcap, 32-bit and 64-bit x86, without remote
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
       GENERATOR: "Visual Studio 17 2022"

CMakeLists.txt

@@ -656,8 +656,8 @@ endif(WIN32)
 if(MSVC)
     add_definitions(-D__STDC__)
     add_definitions(-D_CRT_SECURE_NO_WARNINGS)
-    if(NOT DEFINED PopulationCount64)
-        add_compile_definitions(PopulationCount64=${PopulationCount64})
+    if(DEFINED POPULATIONCOUNT64)
+        add_compile_definitions(PopulationCount64=${POPULATIONCOUNT64})
     endif()
 endif(MSVC)