MinGW: local libgetopt.a is not needed

[eblanca]

I just built vorbis-tools-1.4.0 in Windows environment (all but ogg123) using a recent MinGW (not the latest MinGW-W64). Anyway, the shipped getopt library is no longer needed because it's been a while getopt.h is provided by mingw also, and the standard link library include these functions as well. Further, a compile including the shipped getopt.h generates several warnings due to unmatching argument types in getopt_long, but a compile against the mingw one does not. Of course, I cannot tell whether this shipped getopt module is still needed for Visual Studio or other OSs. Just for you to know.