Fix build with GCC15

[tm-drtina]

GCC 15 changes the default language version for C compilation from -std=gnu17 to -std=gnu23.

Building the cbits with C23 breaks bools:

> c2hs: C header contains errors:
>
> /nix/store/n0zhrw9b9jky2fb3x4dr030agmw8izd7-grpc-1.76.0/include/grpc/credentials.h:231: (column 3) [ERROR]  >>> Syntax error !
>   The symbol `bool' does not fit here.

We already set CC-Options: -std=c99 but c2hs doesn't respect that and uses CPP-Options instead :(