Force C standard to gnu99 in cmake builds.

The Makefiles forces gnu99 since ages. Additionally gcc15 started to
default to C23 and our code doesn't (yet) build with it. Reported in
issue #20.

Author: Yamagi

CMakeLists.txt

@@ -13,10 +13,11 @@ project(yquake2-ctf)
 
 # Enforce compiler flags (GCC / Clang compatible, yquake2
 # won't build with another compiler anyways)
+#  -std=gnu99           -> Force same C standard as in the Makefile
 #  -Wall                -> More warnings
 #  -fno-strict-aliasing -> Quake 2 is far away from strict aliasing
 #  -fwrapv              -> Make signed integer overflows defined
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -fno-strict-aliasing -fwrapv")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99 -Wall -fno-strict-aliasing -fwrapv")
 
 # Use -O2 as maximum optimization level. -O3 has it's problems with yquake2.
 string(REPLACE "-O3" "-O2" CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")