Problem: value for _WIN32_WINNT not always known in CMakeLists.txt

Solution: calculate from CMAKE_SYSTEM_VERSION

Problem: CMAKE_SYSTEM_VERSION might be newer than Windows SDK Version

Solution: limit _WIN32_WINNT value to Visual Studio default Windows SDK version

CMakeLists.txt

@@ -264,9 +264,51 @@ if(NOT CYGWIN)
   check_include_files(windows.h ZMQ_HAVE_WINDOWS)
 endif()
 
+if (WIN32)
+    # from https://stackoverflow.com/a/40217291/2019765
+    macro(get_WIN32_WINNT version)
+        if (CMAKE_SYSTEM_VERSION)
+            set(ver ${CMAKE_SYSTEM_VERSION})
+            string(REGEX MATCH "^([0-9]+).([0-9])" ver ${ver})
+            string(REGEX MATCH "^([0-9]+)" verMajor ${ver})
+            # Check for Windows 10, b/c we'll need to convert to hex 'A'.
+            if ("${verMajor}" MATCHES "10")
+                set(verMajor "A")
+                string(REGEX REPLACE "^([0-9]+)" ${verMajor} ver ${ver})
+            endif ("${verMajor}" MATCHES "10")
+            # Remove all remaining '.' characters.
+            string(REPLACE "." "" ver ${ver})
+            # Prepend each digit with a zero.
+            string(REGEX REPLACE "([0-9A-Z])" "0\\1" ver ${ver})
+            set(${version} "0x${ver}")
+        endif(CMAKE_SYSTEM_VERSION)
+    endmacro(get_WIN32_WINNT)
+
+    get_WIN32_WINNT(ZMQ_WIN32_WINNT_DEFAULT)
+    message(STATUS "Detected _WIN32_WINNT from CMAKE_SYSTEM_VERSION: ${ZMQ_WIN32_WINNT_DEFAULT}")
+
+    # TODO limit _WIN32_WINNT to the actual Windows SDK version, which might be different from the default version installed with Visual Studio
+    if(MSVC_VERSION STREQUAL "1500" AND CMAKE_SYSTEM_VERSION VERSION_GREATER "6.0")
+        set(ZMQ_WIN32_WINNT_LIMIT "0x0600")
+    elseif(MSVC_VERSION STREQUAL "1600" AND CMAKE_SYSTEM_VERSION VERSION_GREATER "6.1")
+        set(ZMQ_WIN32_WINNT_LIMIT "0x0601")
+    elseif(MSVC_VERSION STREQUAL "1700" AND CMAKE_SYSTEM_VERSION VERSION_GREATER "6.1")
+        set(ZMQ_WIN32_WINNT_LIMIT "0x0601")
+    elseif(MSVC_VERSION STREQUAL "1800" AND CMAKE_SYSTEM_VERSION VERSION_GREATER "6.2")
+        set(ZMQ_WIN32_WINNT_LIMIT "0x0602")
+    endif()
+    if(ZMQ_WIN32_WINNT_LIMIT)
+        message(STATUS "Mismatch of Visual Studio Version (${MSVC_VERSION}) and CMAKE_SYSTEM_VERSION (${CMAKE_SYSTEM_VERSION}), limiting _WIN32_WINNT to ${ZMQ_WIN32_WINNT_LIMIT}, you may override this by setting ZMQ_WIN32_WINNT")
+        set(ZMQ_WIN32_WINNT_DEFAULT "${ZMQ_WIN32_WINNT_LIMIT}")
+    endif()
+
+    set(ZMQ_WIN32_WINNT "${ZMQ_WIN32_WINNT_DEFAULT}" CACHE STRING "Value to set _WIN32_WINNT to for building [default=autodetect from build environment]")
+
+    add_definitions(-D_WIN32_WINNT=${ZMQ_WIN32_WINNT})
+endif(WIN32)
+
 if(CMAKE_SYSTEM_NAME STREQUAL "WindowsStore" AND CMAKE_SYSTEM_VERSION STREQUAL "10.0")
   set(ZMQ_HAVE_WINDOWS_UWP ON)
-  add_definitions(-D_WIN32_WINNT=_WIN32_WINNT_WIN10)
 endif()
 if(NOT MSVC)
   check_include_files(ifaddrs.h ZMQ_HAVE_IFADDRS)