It is not the job of a generic build script to decide which version(s) of an OS we are compiling for.

where should it be defined then ?

It should be defined by the parent project.

Example 1: I want to use the default _WIN32_WINNT value provided by my Windows SDK

project(MyProject)
add_executable(MyProject main.cpp)

add_subdirectory(external/miniupnp/miniupnpc)
target_link_libraries(MyProject PRIVATE libminiupnpc-static)

Example 2: I want to specify a common _WIN32_WINNT value across my project (using add_definitions)

project(MyProject)
add_executable(MyProject main.cpp)

add_definitions(-D_WIN32_WINNT=0x0600)

add_subdirectory(external/miniupnp/miniupnpc)
target_link_libraries(MyProject PRIVATE libminiupnpc-static)

Example 3: I want to specify a _WIN32_WINNT value for miniupnpc (using target_compile_definitions)

project(MyProject)
add_executable(MyProject main.cpp)

add_subdirectory(external/miniupnp/miniupnpc)
target_compile_definitions(libminiupnpc-static PRIVATE _WIN32_WINNT=0x0600)
target_link_libraries(MyProject PRIVATE libminiupnpc-static)

Examples 2 and 3 work with master but the defines will have two _WIN32_WINNT values : -D_WIN32_WINNT=0x0501 -D_WIN32_WINNT=0x0600.

The main issue I have is when I want to build a UWP project using cmake [...] -DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10.0.22000.0. In this case, I want to use the default _WIN32_WINNT value (which is not 0x0501). Without this PR, I have to use the following "trick":

if(WINDOWS_STORE)
    get_target_property(miniupnpc-private-defs miniupnpc-private INTERFACE_COMPILE_DEFINITIONS)
    list(REMOVE_ITEM miniupnpc-private-defs "_WIN32_WINNT=0x0501")
    set_property(TARGET miniupnpc-private PROPERTY INTERFACE_COMPILE_DEFINITIONS ${miniupnpc-private-defs})
endif()

It should be defined by the parent project.

so a parent project is mandatory ???

Not necessarily. A target (like an executable) in the same project would be ok too.

Without a target, maybe one can set a CMAKE_C_FLAGS when using the cmake command line to set the value for _WIN32_WINNT but I don't like it.

I have also this option, used by curl:

  set(CURL_TARGET_WINDOWS_VERSION "" CACHE STRING "Minimum target Windows version as hex string")
  if(CURL_TARGET_WINDOWS_VERSION)
    add_definitions(-D_WIN32_WINNT=${CURL_TARGET_WINDOWS_VERSION})
    [...]
  endif()

https://github.com/curl/curl/blob/9e75932358fa24b8a49b7b13202e866d6e395a79/CMakeLists.txt#L110C4-L112

Then, it can be used like this to force the _WIN32_WINNT value instead of using the default one from the SDK:

cmake [...] -DCURL_TARGET_WINDOWS_VERSION=0x0501

I don't mind keeping 0x0501 as the default value to maintain compatibility.

# XP or higher for getnameinfo and friends
set(MINIUPNPC_TARGET_WINDOWS_VERSION "0x0501" CACHE STRING "Minimum target Windows version as hex string")
if(MINIUPNPC_TARGET_WINDOWS_VERSION )
  target_compile_definitions(miniupnpc-private INTERFACE _WIN32_WINNT=${MINIUPNPC_TARGET_WINDOWS_VERSION}) 
endif()

What do you think?

@scribam that's OK if it allows the _WIN32_WINNT value to be se from parent project, but defaults to 0x501 when there is no parent project.

Done