We have required C99 for a while; in the meantime, most C toolchains
have moved on to C11 or later as the default C standard.
We still allow for C99 toolchains, but in the future we are going to
require a C11 toolchain to build and use GLib.
I'm not aware of any platforms where this is a problem in practice, but
it's definitely nonportable and doesn't hurt to document it.
I wonder about CHERI....
GTK lost it's '+' suffix back in 2019, according to
<https://mail.gnome.org/archives/gtk-devel-list/2019-February/msg00000.html>
This commit can be re-generated with:
git grep -l GTK+ \
| grep -v -e ^NEWS -e ^glib/tests/collate.c \
| xargs sed -i 's/GTK+/GTK/g'
Most of the changes are in comments and documentation.
We have fallback in places for GNU's variadic arguments in macros, and
for static inline functions with variadic arguments as an fallback of
last resort, but going forward we are going to depend on `__VA_ARGS__`
for macros that cannot be re-implemented using a static inline function.
Fixes: #2681
