Describe the bug

If the package pkg-config is not installed in the system, the build process silently builds redis without systemd support even though it would be available.

If one explicitly sets USE_SYSTEMD=yes and pkg-config is not available, then the build script says that libsystemd could not be found which is very confusing when knowing it actually is there.

To reproduce

run make on a Ubuntu-based system without pkg-config installed

Expected behavior

throw error that pkg-config is not installed

On top of that, what probably is covered by #8036, when running redis not built with systemd, it's not clear this is causing issues when still trying to use it with systemd. There is no indication in the log about this.
(I wasted a lot of time because I was following my own guide I previously made for redis installation on different system where it worked fine with systemd. So figuring out the issue happened at built time due to missing pkg-config and is not a config issue wasn't very obvious.)

Additional information

Lubuntu 20.04
redis 6.0.9