build: Enable C99 support in meson.build
build: Enable C99 support in meson.build

We use C99 features (tmpl-branch-node.c), so need to explicitly
enable them in meson.build, as some compilers will not enable them
automatically, and will instead error when they encounter usage of C99.

Further, enable gnu11, as per previous patches to other projects, where
Christian wants to also be able to use GNU extensions. (See
https://github.com/chergert/libdazzle/pull/10, for example.)

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

chergert/template-glib,GNOME/template-glib