weston-launch: Provide a default version that doesn't require PAM

weston-launch requires PAM for starting weston as a non-root user.

Since starting weston as root is a valid use case by itself, if
PAM is not available, provide a default version of weston-launch
without non-root-user support.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Ming Liu <ming.liu@toradex.com>