[build] Require C23 (gnu2x)

Now that gcc-13 is default on Debian sid, I'm happy to switch to
requiring it for building QF. As a celebration, I removed the bool and
auto hacks.

config.d/compiling.m4

@@ -320,6 +320,25 @@ if test "x$GCC" != xyes -a "x$CLANG" != xyes -a "x$leave_cflags_alone" != xyes;
   esac
 fi
 
+if test "x$leave_cflags_alone" != xyes; then
+	AC_COMPILE_IFELSE(
+		[AC_LANG_PROGRAM(
+			[[bool flag = true;]],
+			[[return flag ? 1 : 0]])],
+		[],
+		[QF_CC_OPTION(-std=gnu2x)]
+	)
+fi
+AC_MSG_CHECKING([for c23])
+AC_COMPILE_IFELSE(
+	[AC_LANG_PROGRAM(
+		[[bool flag = true;]
+		 [int bar (void);]],
+		[[return bar();]])],
+	[AC_MSG_RESULT(yes)],
+	[AC_MSG_ERROR(QuakeForge requires C23 to compile)]
+)
+
 AS="$CC"
 if test "x$SYSTYPE" = "xWIN32"; then
 	ASFLAGS="\$(DEFS) \$(CFLAGS) \$(CPPFLAGS) \$(DEFAULT_INCLUDES) \$(INCLUDES) -D_WIN32"

include/QF/qtypes.h

@@ -52,25 +52,6 @@
 typedef uint8_t byte;
 #endif
 
-#if __STDC_VERSION__ < 202000 || defined(__clang__)
-# define auto __auto_type
-#endif
-
-#if __STDC_VERSION__ < 202000
-# ifndef _DEF_BOOL_
-#  define _DEF_BOOL_
-// KJB Undefined true and false defined in SciTech's DEBUG.H header
-#  ifdef __cplusplus
-#   define __bool_true_false_are_defined
-#  endif
-#  ifndef __bool_true_false_are_defined
-#   undef true
-#   undef false
-typedef	enum	{false, true} bool;
-#  endif
-# endif
-#endif
-
 // From mathlib...
 typedef float	vec_t;		///< The basic vector component type
 typedef vec_t	vec3_t[3];	///< A 3D vector (used for Euler angles and motion vectors)