[SSSD] [PATCH] Only check systemd unit dir if systemd is selected

[Jakub Hrozek]

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Configure was failing on systems without systemd-units installed because
pkg-config would report an empty string for systemd unit directory.

We should not run any detection of systemd unit dir if systemd is not
selected as the init script type.

This probably slipped through because even F14 has systemd-units package
and the corresponding .pc file.
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