Add time-admin dependency check on systemd

[zhuyaliang]

time-admin depends on systemd. If systemd is not used in the systemd, time-admin is disabled

[zhuyaliang]

@cwendling I am not very familiar with autoconf, can I modify it this way?

[lukefromdc]

Looks to me like we are setting a variable, then setting it to a non-null value only if systemd is detected. I also don't know the build systems that well but this should have the same effect when systemd is not present as the
= \
lines as that's what it should expand to in those cases

We also support meson for mate-control-center, though only one actually HAS to work for non-systemd distros to build m-c-c today, the point to any meson work is an eventual transition to it. That is important especially if autotools ever goes unmaintained.

[zhuyaliang]

After this PR is accepted, I will add meson support for checking systemd

[cwendling]

Almost OK :)
You can also verify it does the right thing if make dist creates a tarball with the time-admin capplet sources when run from a build tree configured with systemd disabled.

[cwendling]

LGTM

[zhuyaliang]

Fix #744