sdeventplus is a c++ wrapper around the systemd sd_event apis meant to provide c++ ergonomics to their usage.
The sdeventplus library requires a libsystemd development package on the system for sd-event.
For a standard release build, you want something like:
meson setup -Dexamples=false -Dtests=disabled builddir ninja -C builddir ninja -C builddir install
For a test / debug build, a typical configuration is
meson setup -Dtests=enabled builddir meson test -C builddir