Build separate static/shared jpeg12/16 obj libs If PIC isn't enabled for the entire build (using CMAKE_POSITION_INDEPENDENT_CODE), then we need to enable it for any of the objects in the libjpeg-turbo shared libraries. Ideally, however, we don't want to enable PIC for any of the objects in the libjpeg-turbo static libraries, unless CMAKE_POSITION_INDEPENDENT_CODE is set. Thus, we need to build separate static and shared jpeg12 and jpeg16 object libraries. Fixes #684