Fix strcasestr redefine warnings for G++

See merge request KartKrew/Kart!844

Fix for GCC C++ compiler, which always defines _GNU_SOURCE.

C++ IO abstractions and std::span

See merge request KartKrew/Kart!840

Add extern "C" in C++ to all headers

See merge request KartKrew/Kart!842

typo 5395194e

QOL fixes for CMake and Git integration

See merge request KartKrew/Kart!838

- Detect and highlight detached HEAD on title screen and in console
- Remove '-' between branch and commit in console

Remote branch name can now be resolved from detached HEAD.

Extra changes for TESTERS builds

See merge request KartKrew/Kart!835

* Create a seperate `unlocks.pk3` so we can customise what the testers have.
* Make TESTERS(/HOSTTESTERS) builds have their own default gamedata.

Special Stages

See merge request KartKrew/Kart!778

Make ring respawn a division

See merge request KartKrew/Kart!833

Add half hitlag to invulnerable players if they would normally take damage

Closes #364

See merge request KartKrew/Kart!814

Fix spring stack overflow

Closes #377

See merge request KartKrew/Kart!831

Alter MF_NOCLIP behavior

See merge request KartKrew/Kart!829

Fix netsave loading crash from saving gamedata

Closes #380

See merge request KartKrew/Kart!832

Add TESTERS and HOSTTESTERS builds to CMake options

See merge request KartKrew/Kart!830

Lets it stay very long for 1v1 like we want, but not matter the closer it gets to 8 players. Tired of people spamming rings to compensate for this problem :V

P_LoadLevel calls some functions that save gamedata and
unset save_p.

Closes #377

Jawz have their old missable handling, but now deal more damage.