Use std=gnu11 (#940)

We already have some C11 pieces like Static_assert and we are likely
going to add more - tell meson to use C11.
Plain C11 does not work as there are several language extension,
like typeof - part of C23 only. We might be able to fix/workaround
such extensions with compiler ifdef, but then I prefer gnu11 over
checking which compiler is used (example gcc has __typeof vs
clangs __typeof++). And compiler extension would also defeat pedantic.

meson.build

@@ -2,6 +2,7 @@ project('libfuse3', ['c'], version: '3.17.0',
         meson_version: '>= 0.51',
         default_options: [
             'buildtype=debugoptimized',
+            'c_std=gnu11',
             'cpp_std=c++11',
             'warning_level=2',
         ])