use pkg-config to find libjpeg
use pkg-config to find libjpeg

fall back to FIND_JPEG if we have to, see

https://github.com/jcupitt/libvips/issues/431

lovell/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips