mad: s/HOMEBREW_PREFIX/prefix/ in pkg-config

This formula generates a mad.pc file for pkg-config. Use #{prefix}
instead of #{HOMEBREW_PREFIX} so dependent formulae using pkg-config
will get the right lib/include paths.
libspotify: s/HOMEBREW_PREFIX/prefix/ in pkg-config

This formula generates a libspotify.pc file for pkg-config. Use #{prefix}
instead of #{HOMEBREW_PREFIX} so dependent formulae using pkg-config
will get the right lib/include paths.
libid3tag: s/HOMEBREW_PREFIX/prefix/ in pkg-config

This formula generates an id3tag.pc file for pkg-config. Use #{prefix}
instead of #{HOMEBREW_PREFIX} so dependent formulae using pkg-config
will get the right lib/include paths.
nss: s/HOMEBREW_PREFIX/prefix/ in pkg-config

This formula generates an nss.pc file for pkg-config. Use #{prefix}
instead of #{HOMEBREW_PREFIX} so dependent formulae using pkg-config
will get the right lib/include paths.
mpd: reorganize deps, options; build bugfix

Add sqlite3 and libsamplerate as :recommends. Remove an unnecessary dep
on mad (ffmpeg can do everything it can); add a dep on faad2.

Make the deps for streaming output (libshout, lame, flac) optional.

Make SoundCloud support an option. Fixes a build bug wherein if yajl was
installed, mpd's configure would half-detect it and then fail to build
because superenv. Now, if --with-yajl specified, depend on yajl
explicitly so it's in the build env; if --without-yajl, tell mpd's
configure to avoid yajl.