During "as-installed" testing, we should search the GIR_DIR for GIR XML,
instead of hard-coding that it is `${prefix}/share/gir-1.0`. This is
not the case on at least Debian, in order to make it possible to
install more than one architecture's flavour of `GLib-2.0.gir`,
which contains some architecture-specific `#define`s.
Also search GOBJECT_INTROSPECTION_DATADIR/GIR_SUFFIX (in practice
something like `/usr/share/gir-1.0` in all cases) to accommodate
distributions like Debian that move the architecture-independent
majority of GIR XML into /usr/share to avoid duplication, leaving
only the architecture-specific minority of files like `GLib-2.0.gir`
in the GIR_DIR.
Signed-off-by: Simon McVittie <smcv@collabora.com>
