Clean up building with USE_SYSTEMD. (#8073)
Clean up building with USE_SYSTEMD. (#8073)

When USE_SYSTEMD=yes is specified, try to use pkg-config to determine
libsystemd linker flags. If not found, silently fall back to simply
using "-lsystemd".

We now use a LIBSYSTEMD_LIBS variable so users can explicitly override
it and specify their own library.

If USE_SYSTEMD is unspecified the old behavior of auto-enabling it if
both pkg-config and libsystemd are available is retained.
JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis