RealtimeKit 0.13

• Add meson build files (autotools stays available for now)
• Replace the use of as with xxd for including a file as a string, which avoids some pitfalls (#15 and portability problems). The tarball contains the resulting file, so xxd is not necessary when building using autotools
• Move rtkit-daemon.service from graphical.target to multi-user.target (#12)
• Make libsystemd an optional dependency (#13)