ISO C allows compilers to make enums smaller than int if their
enumerated values would all fit in the range of a smaller type.

In practice, I suspect that in relevant compilers, all enums whose
values fit in the range INT32_MIN to INT32_MAX (inclusive) are the same
size as int. ISO C allows compiler to break that assumption, but those
that do would break code that works fine in other compilers, making the
compiler look bad, for no significant benefit. I conjecture that such
compilers are not popular.

Let's statically assert that my assumption holds. If all goes well,
GLib will continue to compile on every relevant platform; if it
fails to compile on some platform as a result of this change, then
there are probably a lot of naive uses of enums that need auditing
for this assumption.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=730932


Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Allison Lortie