The sim directory attempts to enable the -Wstrict-prototypes compiler flag.  While it is support in gcc, the flag is not enabled because the config check results in:

configure:16654: ccache gcc -c -g3 -O0 -fsanitize=address -fmax-errors=1 -fdiagnostics-color=always     -Werror -Wstrict-prototypes  co
nftest.c >&5
conftest.c:163:1: error: function declaration isn't a prototype [-Werror=strict-prototypes]
  163 | main ()
      | ^~~~

The main function in the config test is:

| int
| main ()
| {
| 
|   ;
|   return 0;
| }


So yeah, the flag is not enabled because it finds in the config test exactly the kind of things it's meant to find :)