Newer versions of the Windows API may not only add, but also remove API
functions. Therefore, when the user is e.g. building for Windows 10, we
should not set _WIN32_WINNT to e.g. Windows 8, as this may enable the
use of APIs that were present in Windows 8 but removed in Windows 10.
Suggested by Steve Lhomme <robux4@ycbcr.xyz> in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-05/msg00318.html>.
* lib/ftruncate.c (_WIN32_WINNT): Don't set to a smaller value.
* lib/sethostname.c (_WIN32_WINNT): Likewise.
* lib/stat-w32.c (_WIN32_WINNT): Likewise.