cmake: do not force Windows target versions

The goal of this patch is to avoid CMake forcing specific Windows
versions and rely on toolchain defaults or manual selection instead.
This gives back control to the user. This also brings CMake closer to
how autotools and `Makefile.m32` behaves in this regard.

- CMake had a setting `ENABLE_INET_PTON` defaulting to `ON`, which did
  nothing else than fixing the Windows build target to Vista. This also
  happened when the toolchain did not have Vista support (e.g. original
  MinGW), breaking such builds.

  In other environments it did not make a user-facing difference,
  because libcurl has its own pton() implementation, so it works well
  with or without Vista's inet_pton().

  This patch drops this setting. inet_pton() is now used whenever
  building for Vista or newer, either when requested manually or by
  default with modern toolchains (e.g. mingw-w64). Older envs will fall
  back to curl's pton().

  Ref: curl#9027 (comment)
  Ref: curl#8997 (comment)

- When the user did no select a Windows target version manually, stop
  explicitly targeting Windows XP, and instead use the toolchain default.

  This may pose an issue with old toolchains defaulting to pre-XP
  targets. In such case you must manually target Windows XP via:
    `-DCURL_TARGET_WINDOWS_VERSION=0x0501`
  or
    `-DCMAKE_C_FLAGS=-D_WIN32_WINNT=0x0501`

Reviewed-by: Jay Satiro
Reviewed-by: Marcel Raad
Closes curl#9046

CMakeLists.txt

@@ -78,20 +78,11 @@ option(BUILD_SHARED_LIBS "Build shared libraries" ON)
 option(ENABLE_ARES "Set to ON to enable c-ares support" OFF)
 if(WIN32)
   option(CURL_STATIC_CRT "Set to ON to build libcurl with static CRT on Windows (/MT)." OFF)
-  option(ENABLE_INET_PTON "Set to OFF to prevent usage of inet_pton when building against modern SDKs while still requiring compatibility with older Windows versions, such as Windows XP, Windows Server 2003 etc." ON)
   option(ENABLE_UNICODE "Set to ON to use the Unicode version of the Windows API functions" OFF)
   set(CURL_TARGET_WINDOWS_VERSION "" CACHE STRING "Minimum target Windows version as hex string")
   if(CURL_TARGET_WINDOWS_VERSION)
     add_definitions(-D_WIN32_WINNT=${CURL_TARGET_WINDOWS_VERSION})
     set(CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS} -D_WIN32_WINNT=${CURL_TARGET_WINDOWS_VERSION}")
-  elseif(ENABLE_INET_PTON)
-    # _WIN32_WINNT_VISTA (0x0600)
-    add_definitions(-D_WIN32_WINNT=0x0600)
-    set(CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS} -D_WIN32_WINNT=0x0600")
-  else()
-    # _WIN32_WINNT_WINXP (0x0501)
-    add_definitions(-D_WIN32_WINNT=0x0501)
-    set(CMAKE_REQUIRED_DEFINITIONS "${CMAKE_REQUIRED_DEFINITIONS} -D_WIN32_WINNT=0x0501")
   endif()
   if(ENABLE_UNICODE)
     add_definitions(-DUNICODE -D_UNICODE)