Do not use internal libjpeg.

Do not use internal libjpeg.

Nowadays, OpenMotif is itself linked to libjpeg, so pulling in another
version of it causes symbol size mismatches, not to mention the maintenance
burden and security implications arising from keeping our own copy of libjpeg.

We still need some of the header files provided here because they are internal
to libjpeg and not installed on most distributions.