Set -D_WIN32_WINNT for static deps; unbound, in particular, needs this
as the latest version appears to rely on something only provided in
non-ancient windows to build properly.
This required moving _winver into the toolchain file so that it is
available earlier in cmake code (StaticBuild is included long before
win32.cmake), but also this seems a more appropriate place for it.
We set CMAKE_CROSSCOMPILE in a few places, but that is just a typo for
CMAKE_CROSSCOMPILING. However we don't need to set it at all: cmake
does so automatically whenever CMAKE_SYSTEM_NAME is set (which is why
our cross compiling actually works currently.)