Freeciv

#error in version.h should be just a warning, but it was not possible at the time it was added. Since c23 the #warning directive exist. Turn the #error to #warning.

pre-c23 compilers: They fail either way, but for a different reason (#error directive instructs them to fail / they don't understand #warning) and error message is not necessarily as immediately clear as before (but one checking the failing code line sees the message there)

c23 compatible compilers: They start to just warn instead of outright failing. Note that -Werror might turn warning to an error after all.