MINGW64
- uses 0x601 as value for _WIN32_WINNT. No need for us to define
it to that value or even lower values in some places.
This also gets rid of:  warning: "_WIN32_WINNT" redefined
- has 0x0502 for WINVER, so get rid of us setting it to 0x0500 in
gimp-app-test-utils.h. It also seems that the need to use G_OS_WIN32
has disappeared here.
- DIRECTINPUT_VERSION is 0x0800, no need for us to set it to that value.
- AI_ADDRCONFIG was apparently missing from the MINGW headers in the
past, but not anymore.