libmad: Add optimization config options

Configuring libmad with --enable-speed compromises accuracy and can cause audio
clipping in some cases (heard on ARM platform with some loud MP3s), so give
users the choice of MAD optimizations.

The default config corresponds to the default behavior of libmad's configure.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>