cmake: ZMQ_WIN32_WINNT autodetection broken when cross-compiling with mingw-w64

[fanquake]

When cross-compiling for Windows on Ubuntu, using mingw-w64, the functionality in the CMake build:

Value to set _WIN32_WINNT to for building [default=autodetect from build environment]

does not work. After configuring with the following:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release \
-DCMAKE_SYSTEM_NAME=Windows \
-DCMAKE_C_COMPILER=x86_64-w64-mingw32-gcc \
-DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-g++

<snip>
-- CURVE security is disabled
-- Detected _WIN32_WINNT from CMAKE_SYSTEM_VERSION: 
-- Looking for kqueue
<snip>

cmake --build build -j$(nproc) --verbose
....
[  2%] Building CXX object CMakeFiles/libzmq.dir/src/ws_engine.cpp.obj
In file included from /root/libzmq/src/precompiled.hpp:22,
                 from /root/libzmq/src/ws_encoder.cpp:3:
/root/libzmq/src/windows.hpp:21:19: error: operator '>=' has no left operand
   21 | #if (_WIN32_WINNT >= 0x0501)
      |                   ^~
/root/libzmq/src/windows.hpp:23:2: error: #error You need at least Windows XP target
   23 | #error You need at least Windows XP target
      |  ^~~~~
/usr/bin/x86_64-w64-mingw32-g++ -DFD_SETSIZE=16384 -DZMQ_BUILD_DRAFT_API -DZMQ_CUSTOM_PLATFORM_HPP -DZMQ_STATIC -D_CRT_SECURE_NO_WARNINGS -D_REENTRANT -D_THREAD_SAFE -D_WIN32_WINNT="" -D_WINSOCK_DEPRECATED_NO_WARNINGS @CMakeFiles/libzmq-static.dir/includes_CXX.rsp -std=c++11 -Wno-tautological-constant-compare -O3 -DNDEBUG -MD -MT CMakeFiles/libzmq-static.dir/src/ws_address.cpp.obj -MF CMakeFiles/libzmq-static.dir/src/ws_address.cpp.obj.d -o CMakeFiles/libzmq-static.dir/src/ws_address.cpp.obj -c /root/libzmq/src/ws_address.cpp
/usr/bin/x86_64-w64-mingw32-g++ -DFD_SETSIZE=16384 -DZMQ_BUILD_DRAFT_API -DZMQ_CUSTOM_PLATFORM_HPP -DZMQ_STATIC -D_CRT_SECURE_NO_WARNINGS -D_REENTRANT -D_THREAD_SAFE -D_WIN32_WINNT="" -D_WINSOCK_DEPRECATED_NO_WARNINGS @CMakeFiles/libzmq-static.dir/includes_CXX.rsp -std=c++11 -Wno-tautological-constant-compare -O3 -DNDEBUG -MD -MT CMakeFiles/libzmq-static.dir/src/ws_connecter.cpp.obj -MF CMakeFiles/libzmq-static.dir/src/ws_connecter.cpp.obj.d -o CMakeFiles/libzmq-static.dir/src/ws_connecter.cpp.obj -c /root/libzmq/src/ws_connecter.cpp
/usr/bin/x86_64-w64-mingw32-g++ -DFD_SETSIZE=16384 -DZMQ_BUILD_DRAFT_API -DZMQ_CUSTOM_PLATFORM_HPP -DZMQ_STATIC -D_CRT_SECURE_NO_WARNINGS -D_REENTRANT -D_THREAD_SAFE -D_WIN32_WINNT="" -D_WINSOCK_DEPRECATED_NO_WARNINGS @CMakeFiles/libzmq-static.dir/includes_CXX.rsp -std=c++11 -Wno-tautological-constant-compare -O3 -DNDEBUG -MD -MT CMakeFiles/libzmq-static.dir/src/ws_encoder.cpp.obj -MF CMakeFiles/libzmq-static.dir/src/ws_encoder.cpp.obj.d -o CMakeFiles/libzmq-static.dir/src/ws_encoder.cpp.obj -c /root/libzmq/src/ws_encoder.cpp
/usr/bin/x86_64-w64-mingw32-g++ -DDLL_EXPORT -DFD_SETSIZE=16384 -DZMQ_BUILD_DRAFT_API -DZMQ_CUSTOM_PLATFORM_HPP -D_CRT_SECURE_NO_WARNINGS -D_REENTRANT -D_THREAD_SAFE -D_WIN32_WINNT="" -D_WINSOCK_DEPRECATED_NO_WARNINGS -Dlibzmq_EXPORTS @CMakeFiles/libzmq.dir/includes_CXX.rsp -std=c++11 -Wno-tautological-constant-compare -O3 -DNDEBUG -MD -MT CMakeFiles/libzmq.dir/src/ws_decoder.cpp.obj -MF CMakeFiles/libzmq.dir/src/ws_decoder.cpp.obj.d -o CMakeFiles/libzmq.dir/src/ws_decoder.cpp.obj -c /root/libzmq/src/ws_decoder.cpp
In file included from /usr/share/mingw-w64/include/windows.h:10,
                 from /usr/share/mingw-w64/include/winsock2.h:23,
                 from /root/libzmq/src/windows.hpp:27:
/usr/share/mingw-w64/include/sdkddkver.h:192:19: error: operator '<=' has no left operand
  192 | #if (_WIN32_WINNT <= _WIN32_WINNT_NT4)
      |                   ^~
/usr/share/mingw-w64/include/sdkddkver.h:194:21: error: operator '<=' has no left operand
  194 | #elif (_WIN32_WINNT <= _WIN32_WINNT_WIN2K)
      |                     ^~
/usr/share/mingw-w64/include/sdkddkver.h:196:21: error: operator '<=' has no left operand
  196 | #elif (_WIN32_WINNT <= _WIN32_WINNT_WINXP)
      |                     ^~
/usr/share/mingw-w64/include/sdkddkver.h:198:21: error: operator '<=' has no left operand
  198 | #elif (_WIN32_WINNT <= _WIN32_WINNT_WS03)
      |                     ^~
/usr/share/mingw-w64/include/sdkddkver.h:200:21: error: operator '<=' has no left operand
  200 | #elif (_WIN32_WINNT <= _WIN32_WINNT_VISTA)
      |                     ^~
/usr/share/mingw-w64/include/sdkddkver.h:202:21: error: operator '<=' has no left operand
  202 | #elif (_WIN32_WINNT <= _WIN32_WINNT_WIN7)
      |                     ^~
/usr/share/mingw-w64/include/sdkddkver.h:204:21: error: operator '<=' has no left operand
  204 | #elif (_WIN32_WINNT <= _WIN32_WINNT_WIN8)
      |                     ^~
/usr/share/mingw-w64/include/sdkddkver.h:215:61: error: operator '!=' has no left operand
  215 | #if ((OSVER(NTDDI_VERSION) == NTDDI_WIN2K) && (_WIN32_WINNT != _WIN32_WINNT_WIN2K)) || \

After manually setting -DZMQ_WIN32_WINNT=VERSION, things mostly compile as expected (except for a different issue that requires passing -DZMQ_HAVE_IPC=OFF). Althought note that the output is still incorrect, as there mention of autodetection, and still no version printed i.e:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release \
-DCMAKE_SYSTEM_NAME=Windows \
-DCMAKE_C_COMPILER=x86_64-w64-mingw32-gcc \
-DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-g++ \
-DZMQ_WIN32_WINNT=0x0601
<snip>
-- CURVE security is disabled
-- Detected _WIN32_WINNT from CMAKE_SYSTEM_VERSION: 
-- Looking for kqueue

Environment

• libzmq version (commit hash if unreleased): 4a5ece5
• OS: Ubuntu Noble
• GCC x86_64-w64-mingw32-g++ (GCC) 13-win32
• Mingw-w64: 11.0.1

Noticed while porting a project that cross-compiles using Autotools & mingw-w64.