configure: do not default to system libjpeg on Linux

There are multiple different sonames of libjpeg in different distros,
so use a system one only when system libs are requested.

(cherry picked from commit 20d1e20d16ac377c160d47b53ff487a75fbd166f)

Conflicts:
	configure.ac

Change-Id: I164433a31ab0fb4723da6843eb1f1132c8447d9e
Reviewed-on: https://gerrit.libreoffice.org/13157
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>