use own libjpeg-turbo mirror
always include libjpeg for Windows builds (for lossy DNG support)
later we will also need it for rawspeed support
remove support for 3 and 4 color raw images
These were introduced in libraw 0.15 and a conditional inclusion of the
different struct types seems impossible with Cython, therefore, as we
still support 0.14 (Ubuntu 12.04) we skip support for now.