add condition to check CMAKE_SYSTEM_VERSION defined

Author: toge

CMakeLists.txt

@@ -117,7 +117,7 @@ if(BUILD_SHARED_LIBS AND WIN32 AND HTTPLIB_COMPILE)
 	set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
 endif()
 
-if(CMAKE_SYSTEM_NAME MATCHES "Windows" AND ${CMAKE_SYSTEM_VERSION} VERSION_LESS "10.0.0")
+if(CMAKE_SYSTEM_NAME MATCHES "Windows" AND DEFINED CMAKE_SYSTEM_VERSION AND CMAKE_SYSTEM_VERSION VERSION_LESS "10.0.0")
 	message(SEND_ERROR "Windows ${CMAKE_SYSTEM_VERSION} or lower is not supported. Please use Windows 10 or later.")
 endif()
 if(CMAKE_SIZEOF_VOID_P LESS 8)