Add new configure option to set the path to "polkit-agent-helper-1".

The path cannot be obtained from pkg-config and `pkg-config
--variable=prefix polkit-agent-1` is not good enough.

On Fedora, the path is "/usr/lib/polkit-1/polkit-agent-helper-1".
On Debian Buster, the path is "/usr/lib/policykit-1/polkit-agent-helper-1"
On Debian Sid, the path is "/usr/libexec/polkit-agent-helper-1" (but
currently it is also symlinked from "/usr/lib/policykit-1/polkit-agent-helper-1".

(cherry picked from commit 801c41a1)