Fix platform detection for third party libraries

Author: mthrok

src/third_party/CMakeLists.txt

@@ -22,7 +22,7 @@ set(CMAKE_EXPORT_NO_PACKAGE_REGISTRY true)
 
 add_subdirectory(fmt)
 add_subdirectory(glog)
-if (MSVC)
+if (WIN32)
 add_subdirectory(zlib)
-endif (MSVC)
+endif (WIN32)
 add_subdirectory(libdeflate)

src/third_party/ffmpeg/multi/CMakeLists.txt

@@ -131,7 +131,7 @@ elseif (UNIX)
       " and library files are present) will invoke the FFmpeg single-version integration. "
       "If you do not need the FFmpeg integration, setting USE_FFMPEG=0 will bypass the issue.")
   endif()
-elseif(MSVC)
+elseif(WIN32)
     FetchContent_Declare(
       f4
       URL ${base_url}/ffmpeg_4.4.5_win64.tar.gz