GCC 15.1.1 support: `nullptr`

[Robotechnic]

Hello,

I have an issue with using GCC 15.1.1 with Goblint. When I run it on a simple test, I get the following output:

/usr/lib/gcc/x86_64-pc-linux-gnu/15.1.1/include/stddef.h:465: Error: Cannot resolve variable nullptr.
Error: There were parsing errors in .goblint/preprocessed/stdlib.i
Fatal error: exception Goblint_lib__Maingoblint.FrontendError("Errormsg.Error")

Running it with --set exp.cpp-path to use gcc14, works as expected.

[sim642]

This will be very crucial for a new release. Bunch of the CIs in ocaml/opam-repository#28155 now fail for the exact same reason.

[sim642]

This is because GCC 15 defaults to C23: https://gcc.gnu.org/gcc-15/porting_to.html. Perhaps we should work around it by explicitly setting the --std argument all the time. Goblint even has a cil.cstd option with the default value c99 but it's not passed to GCC.

Although CIL tests might also fail because they actually run GCC.