build: pass -fsigned-char on clang

[tobhe]

Powerpc and ARM default to unsigned char which causes an immediate crash when starting the game.
On gcc this is prevented by passing -fsigned-char in CFLAGS and CXXFLAGS. This change does the same for clang.

[xycaleth]

Out of curiosity, where is it crashing specifically when you try to start the game?

[tobhe]

It's crashing right after dropping you into the actual game world when starting a new game. The character selection and cutscenes work.

The actual segfault happens in CFxScheduler::AddScheduledEffects():

OpenJK/code/cgame/FxScheduler.cpp

Line 1403 in d2ed03a

if (cent.gent->ghoul2.IsValid())

The reason seems to be that the previous check in

OpenJK/code/cgame/FxScheduler.cpp

Line 1381 in d2ed03a

else if (effect->mBoltNum == -1)

doesn't work because mBoltNum as a char is unsigned and the compiler decides that this can never be true.

[xycaleth]

This looks good to me. Maybe in the future we can stop relying on char signedness but this is a good quick fix.