A patch from Pascal Stumpf using external jpeg broke linux builds,
since libjpeg needs to be linked in when using a remote jpeg lib.
So, in lnxLib.tmpl, define SharedDtHelpReqs so -ljpeg is used. Also,
in DtHelp/Imakefile, use proper Arch defines so external jpeg libs are
only used on linux, fbsd, and obsd systems.