php: add PACKAGECONFIG to fix implicit dependency on libpam

Add a pam PACKAGECONFIG option defaulted from the pam DISTRO_FEATURES
item, so that if pam isn't in DISTRO_FEATURES, it is explicitly
disabled (and conversely if it is in DISTRO_FEATURES that we have an
explicit dependency on libpam).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>