libsidplayfp: Handle options

Introduce three optimizing options: BHINTS, INLINE, and MMX,
while giving well-known general options: DOCS, DOXYGEN, and DEBUG.
BHINTS   --enable-branch-hints  (default: enabled)
INLINE   --enable-inline        (default: enabled)
MMX      --enable-mmx           (default: disabled)
DEBUG    --enable-debug=full    (default: =no)

See ${DOCSDIR}/README for more information about these options.