configure.ac: use mingw as the case instead of mingw32
configure.ac: use mingw as the case instead of mingw32

config.guess returns x86_64-pc-mingw64 on some msys2 installations

Signed-off-by: Christopher Degawa <3962a0508f18a69b6a96dda4efebf2513ae1e396@randomderp.com>

ShiftMediaProject/libbdplus,ShiftMediaProject/libbdplus,ShiftMediaProject/libbdplus