meson/build: fix using correct prefix for "systemdsystemunitdir"

We do the same with autotools.

Well, almost the same. Of course, meson's define_variable only
accepts a list of two strings, to define one variable. So we cannot
also redefine "prefix", unlike configure.ac.
57 jobs for th/meson-prefix-dir in 32 minutes and 29 seconds (queued for 7 seconds)