This is a step towards #1484. We officially require a C99 toolchain,
so we can statically assert that our artisanal hand-crafted integer
types are compatible with the ones we would like to recommend people
use instead.

If there are *still* platforms where <stdint.h> is problematic, these
static assertions can act as an early-warning that future GLib releases
will make a C99-compliant <stdint.h> a hard requirement, in ways that
are less straightforward to avoid (see #1484 and !1300

).

Signed-off-by: Simon McVittie <smcv@collabora.com>