cups: check PACKAGECONFIG for pam feature

cups: check PACKAGECONFIG for pam feature

The cups' PACKAGECONFIG is populated based on DISTRO_FEATURES, but a user
is free to enable or disable PACKAGECONFIGs at will. In theory it is
possible that pam is enabled globally in DISTRO_FEATURES but disabled in
cups' PACKAGECONFIG. Checking the PACKAGECONFIG to determine whether or not
pam is enabled would be a safer check rather than relying on DISTRO_FEATURES.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>