meson: build libsystemd-core via an intermediate static library

meson: build libsystemd-core via an intermediate static library

By itself, this is not useful. I'm making this a separate commit to
make debugging easier. It turns out that meson does static libraries
using references, so the "static library" a tiny stub stub that refers
to the object files on disk and this has negligible cost:
$ ls -lhd build/src/core/libsystemd-core-257.{a,so}
-rw-r--r-- 1 zbyszek zbyszek  36K Jul  3 16:54 build/src/core/libsystemd-core-257.a
-rwxr-xr-x 1 zbyszek zbyszek 6.1M Jul  3 16:54 build/src/core/libsystemd-core-257.so