test global _WIN32_WINNT=0x0600

We did not set an explicit Windows target version previously. This
did not cause issues with the default configuration, however it seems
to be a good idea to set it explititly to the Vista, to make sure to
live up to our compatibility goal even if curl or a dependency decides
to enable features specific to newer Windows versions.

curl (as of v8.4.0) doesn't utilize Windows features beyond Vista.

Before this patch targets were defined by the toolchain defaults:
- x64, x86: 0x0a00 (Win10)
- arm64: 0x0601 (Win7) (requires Win10 anyway for the CPU)

_build.sh

@@ -723,6 +723,9 @@ build_single_target() {
   unset CC
 
   if [ "${_OS}" = 'win' ]; then
+
+    _CPPFLAGS_GLOBAL="${_CPPFLAGS_GLOBAL} -D_WIN32_WINNT=0x0600"  # Windows Vista
+
     if [ "${_HOST}" != "${_OS}" ]; then
       _CMAKE_GLOBAL="-DCMAKE_SYSTEM_NAME=Windows ${_CMAKE_GLOBAL}"
     fi