meson: specify rpath on all executables linking to libshared

meson: specify rpath on all executables linking to libshared

(i.e. pretty much all of them). libtools does this automatically for us.  It
would be great if meson did too. Otherwise, it would be nice to simply attach
the information about necessary rpath to the library object, and have it used
automatically for all users. Dunno, maybe there's already a way to do this.

I used '$ORIGIN' for stuff installed into libexecdir, since that's where
libshared ends up, and a full path for everything else. The advantage of
using $ORIGIN is that those binaries can be started from $DESTDIR, which can
sometimes be useful.