allow Mac OS compiling with jpeg-turbo
Currently, the Mac OS X version doesn’t build with jpeg-turbo, but only jpeg when using dynamic linking (the full static might still work).
Tried to fix this issue in 239d57bf3e493d30e266962c041d68dd2ceaf284, but it seems it still doesn’t work.
misc: fixed JPEGTURBO detection on Mac OS X
misc: updated Mac OS X compile instructions