Modern C and compatibility fixes

[MisterDA]

This PR:

• Fixes modern C89/C99/C11/C17 compilers requiring (void) (no parameter) instead of () (pre C89 any parameter);
• Fixes mincore detection on BSD and macOS;
• Fixes a compatibility issue with caml_convert_flag_list taking an int * in OCaml 4 and a const int * in OCaml 5, with a minor header reordering;
• Fixes a sign comparison warning.