Wine is now able to use mingw-w64 to build components. When mingw-w64
is installed on FreeBSD, users might inadvertedly use that when we have
not set up things properly yet (and it probably should be an option to
choose, at least initially). For example, mingw-w64 produces .dll files
instead of the current .dll.so files, breaking pkg-plist.

So for now explicitly disable the use of mingw-w64. In a next step,
once mingw-w64 is available, we probably should make this an option.

PR: 237213
Reported by: Damjan Jovanovic <damjan.jov@gmail.com>
MFH: 2020Q4 (blanket: build issue)