Fix 183 libjpeg

Closes #183 -- option to use the system's libjpeg, using -DUSE_SYSTEM_LIBJPEG=ON

As the system's libjpeg is probably newer, some code modifactions were necessary, but they are guarded
with #ifdef USE_NEWER_JPEG.

NOTE: This patch will only apply after merging #191 due to the changes in neo/CMakeLists.txt.