System JPEG builds include in-tree jpeglib.h, resulting in build failure

The header files from the in-tree copy of the JPEG library are included in building jpegdecoder.o, resulting in symbols like jStdError being referenced rather than jpeg_std_error and linking against the system libjpeg then fails.

It seems build logic is present to make this line optional, but it is not being used. Unfortunately, due to the movement of various makefiles, I can't trace when this change was made.