desklock - make it fail to lock on non-bsd platforms if no pam support

if no pam support detected at build time, dont even lock to begin
with. this should fix T6779

yes - even better is making pam a requirement for building and to be
optionallhy disabled, but then even if disabled we still need this check.

this has been a long long long standing bug in e. i think this is why
we had some reports of "any password unlocks e" - because it was built
without pam support and before auth was always succeeding without pam.
now it is always FAILING instead (which is probably better).

@fix

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