We already use several GCC extenions, like typeof() and
__attribute__((cleanup)). They are too convinient to miss
and every supported compiler must support these.
Currently, gcc and clang does. Maybe other compilers would
support that too, but who knows, nobody seems to test that.

We also already use stdbool.h (C99) and the imported systemd
code is mostly gnu99 too (it's not clear to me, because I don't
find it precisely documented. Certainly it makes use of C99 features
too).

C99/gnu99 has some nice improvements that we no longer should miss
out. For example "flexible array members" or "variable declaration
in init-part of for loop".

It doesn't mean we have to use every obscure (badly supported?)
feature, it means we don't have to forgo features that are well
supported. C99 is 17 years old, I mean, really...

If somebody comes along and ports NM to non-gcc/clang, we can address
bugs about unsupported language features as they surface.
But let's not restrict us to some hypothetical compiler (or language
specification).

Also, NetworkManager is not ported on environment beside Linux.
We don't have to be so considerate about the required build environment.
Gcc is probably the most portable compiler out there. I doubt porting
NetworkManager to *BSD fails due to missing gnu99 features. And if that
causes issues, we should fix them after they happen in practice.