If you specify -target x86_64-windows-gnu -lc then some libc functions are provided by Zig, some are provided by vendored mingw-w64 C files, and you don't need mingw-w64 installed separately; Zig provides everything.
You can still pass --libc libc.txt to link against an externally provided libc, such as a separate mingw-w64 installation you have lying around, or even your own libc installation if you want to mess around with that.
If you specify -target x86_64-windows-gnu -lc then some libc functions are provided by Zig, some are provided by vendored mingw-w64 C files, and you don't need mingw-w64 installed separately; Zig provides everything.
You can still pass --libc libc.txt to link against an externally provided libc, such as a separate mingw-w64 installation you have lying around, or even your own libc installation if you want to mess around with that.
Both situations unchanged.