Only use the hard-coded build-time path given by X11_PREFIX on X11 and
Wayland where a X11 package is normally available.  On other platforms,
get the datadir of the running system and mimic the behavior by
constructing the path dynamically.  This avoids hardcoding the path for
searching for compose tables where we want to have relocatability.

This fixes the build on Windows/MSVC as well, where we don't normally have
any X11 packages available.

https://bugzilla.gnome.org/show_bug.cgi?id=757984