mingw: Build autotools opusfile in a subdirectory.
mingw: Build autotools opusfile in a subdirectory.

Invoking ../configure from the mingw directory would clobber
the mingw-specific Makefile, which is confusing. Instead
have the `make opusfile` target invoke the autotools build
within a subdirectory.

xiph/opusfile,xiph/opusfile,xiph/opusfile