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

weston: Provide a default version of weston-launch 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, we check
the distro for 'pam' and build weston-launch with or without
non-root-user support.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>