Unfortunately the format string for a size_t, `%zu`, is not portable
to all Windows compilers, and the appropriate format string for the
fundamental type that implements size_t varies between platforms
(typically `%u` on 32-bit platforms, `%lu` on 64-bit Linux or
`%llu` on 64-bit Windows).
In gtk-demo, cast the number of search results to long, to avoid
breaking up a translatable string.
Elsewhere, use GLib's abstraction for this.
Signed-off-by: Simon McVittie <smcv@debian.org>