build: Optionally require polkit
Polkit has become required since 1696e660 since polkit.its is necessary
for xgettext to parse the .policy
file. This check does not ensure
that polkit.its will be present in all cases, as it may be in a package
without a pkg-config file (example: it is in polkitd on Ubuntu 22.04).
But it should still improve things in general.
Related: !126 (merged)