Correct WinNT driver type when using mingw-w64 on modern versions of Windows

[guy-rouillier]

Building with mingw-w64 7.2.0 on Windows 7 64-bit, the default driver is set to Win32 instead of WinNT; the correct default driver for this version of Windows is WinNT. Update conflib.pl to handle the combination of mingw-w64 7.2.0 and MSYS2. Both the 32-bit and 64-bit versions of MSYS2 report uname MSYS_NT-6.1.

Attached is diff for conflib.pl.
conflib-diff.txt

[set-soft]

Be careful, those drivers aren't that different.
Their name are for historic reasons, not really related to Windows NT vs Windows 95 differences.
Both are good for any Win32 target.